added first version of upgrade guide

docs/upgrade-guide.md

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+---
+layout: page
+title: Upgrade Guide
+---
+
+#### Migration from v2.4.x
+The first step to migrate AdminLTE v2.4.x to v3.0 is upgrade the Bootstrap 3 base code to Bootstrap 4, the full instruction <a href="https://getbootstrap.com/docs/4.3/migration/">here</a> after you upgraded the base code you need to update the markups.
+
+##### Main Header
+The biggest change in Main Header is the Logo is moved to Main Sidebar and the Main Header has now color variations, here are all changes:
+
+1. Logo 
+  - `<a href="index2.html" class="logo">` moved & rebuild to `.brand-link` inside `.main-sidebar`
+2. Header / Nav
+  - `<header class="main-header">` & `<nav class="navbar navbar-static-top">` merged with `<nav class="main-header navbar navbar-expand navbar-white navbar-light">`
+3. Sidebar Toggle / Left Navbar
+  - `<a href="#" class="sidebar-toggle" data-toggle="push-menu" role="button"><span class="sr-only">Toggle navigation</span></a>` replaced with `<ul class="navbar-nav"><li class="nav-item"><a class="nav-link" data-widget="pushmenu" href="#"><i class="fas fa-bars"></i></a></li></ul>`
+3. Right Navbar
+  - `<div class="navbar-custom-menu">` & `<ul class="nav navbar-nav">` merged with `<ul class="navbar-nav ml-auto">`
+
+##### Main Sidebar
+Like above the biggest change is the Main Sidebar contains now the Logo and the sidebar has now color variations, here all changes:
+
+1. Main Sidebar Color
+  - `<aside class="main-sidebar">` replaced with `<aside class="main-sidebar sidebar-dark-primary">`
+2. Logo / Brand Link
+  - `<a href="index3.html" class="brand-link"><img src="dist/img/AdminLTELogo.png" alt="AdminLTE Logo" class="brand-image img-circle elevation-3" style="opacity: .8"><span class="brand-text font-weight-light">AdminLTE 3</span></a>` replaces the old logo
+3. Sidebar
+  - `<section class="sidebar">` replaced with `<div class="sidebar">`
+4. User Panel
+  - `<div class="user-panel"><div class="pull-left image"><img src="dist/img/user2-160x160.jpg" class="img-circle" alt="User Image"></div><div class="pull-left info"><p>Alexander Pierce</p><a href="#"><i class="fa fa-circle text-success"></i> Online</a></div></div>` replaced with `<div class="user-panel mt-3 pb-3 mb-3 d-flex"><div class="image"><img src="dist/img/user2-160x160.jpg" class="img-circle elevation-2" alt="User Image"></div><div class="info"><a href="#" class="d-block">Alexander Pierce</a></div></div>`
+5. Sidebar Menu
+  - `<nav class="mt-2">` now around `<ul class="sidebar-menu" data-widget="tree">`
+  - `<ul class="sidebar-menu" data-widget="tree">` replaced with `<ul class="nav nav-pills nav-sidebar flex-column" data-widget="treeview" role="menu">`
+  - `<li class="header">` repalced with `<li class="nav-header">`
+  - `<li>` need a new class `.nav-item`
+  - `<li> <a>` need a new class `.nav-link`
+  - `<li> <a> <i>` need a new class `.nav-icon`
+  - `<li> <a> <span>` replaced with `<p>`
+  - `<span class="pull-right-container">` removed
+  - `<i class="fa fa-angle-left pull-right"></i>` replaced with `<i class="right fas fa-angle-left"></i>`
+  - `<small class="label pull-right bg-green">new</small>` replaced with `<span class="right badge badge-danger">New</span>`
+  - `<li> <ul class="treeview-menu">` replaced with `<ul class="nav nav-treeview">`
+
+
+<div class="row">
+  <div class="col-md-6" markdown="1">
+Old sample entry
+```html
+<li>
+  <a href="pages/widgets.html">
+    <i class="fa fa-th"></i> <span>Widgets</span>
+    <span class="pull-right-container">
+      <small class="label pull-right bg-green">new</small>
+    </span>
+  </a>
+</li>
+```
+  </div>
+  <div class="col-md-6" markdown="1">
+New sample entry 
+```html
+<li class="nav-item">
+  <a href="pages/widgets.html" class="nav-link">
+    <i class="nav-icon fas fa-th"></i>
+    <p>
+      Widgets
+      <span class="right badge badge-danger">New</span>
+    </p>
+  </a>
+</li>
+```
+  </div>
+</div>
+
+<div class="row">
+  <div class="col-md-6" markdown="1">
+Old sample entry (with tree menu)
+```html
+<li class="treeview">
+  <a href="#">
+    <i class="fa fa-dashboard"></i> <span>Dashboard</span>
+    <span class="pull-right-container">
+      <i class="fa fa-angle-left pull-right"></i>
+    </span>
+  </a>
+  <ul class="treeview-menu">
+    <li class="active"><a href="index.html"><i class="fa fa-circle-o"></i> Dashboard v1</a></li>
+  </ul>
+</li>
+```
+  </div>
+  <div class="col-md-6" markdown="1">
+New sample entry (with tree menu)
+```html
+<li class="nav-item has-treeview">
+  <a href="#" class="nav-link">
+    <i class="nav-icon fas fa-tachometer-alt"></i>
+    <p>
+      Dashboard
+      <i class="right fas fa-angle-left"></i>
+    </p>
+  </a>
+  <ul class="nav nav-treeview">
+    <li class="nav-item">
+      <a href="index.html" class="nav-link active">
+        <i class="far fa-circle nav-icon"></i>
+        <p>Dashboard v1</p>
+      </a>
+    </li>
+  </ul>
+</li>
+```
+  </div>
+</div>
+
+##### Content Header
+The biggest change in content header is AdminLTE use here now `.container-fluid`, `.row` & `.col-*` and the breadcrumb markup changed, here are all changes:
+- `<section class="content-header">` replaced with `<div class="content-header">`
+- `<div class="container-fluid">` added in `<div class="content-header">`
+- `<h1>` & `<ol class="breadcrumb">` rebuild in `<div class="row">` & `<div class="col-sm-6">`
+- `<h1>` replaced with `<h1 class="m-0 text-dark">`
+- `<ol class="breadcrumb">` need new class `.float-sm-right`
+- `<ol class="breadcrumb"> <li>` need new class `.breadcrumb-item`
+
+
+<div class="row">
+  <div class="col-md-6" markdown="1">
+Old Content Header Markup
+```html
+<section class="content-header">
+  <h1>
+    Dashboard
+    <small>Control panel</small>
+  </h1>
+  <ol class="breadcrumb">
+    <li><a href="#"><i class="fa fa-dashboard"></i> Home</a></li>
+    <li class="active">Dashboard</li>
+  </ol>
+</section>
+```
+  </div>
+  <div class="col-md-6" markdown="1">
+New Content Header Markup
+```html
+<div class="content-header">
+  <div class="container-fluid">
+    <div class="row mb-2">
+      <div class="col-sm-6">
+        <h1 class="m-0 text-dark">
+          Dashboard
+          <small>Control panel</small>
+        </h1>
+      </div>
+      <div class="col-sm-6">
+        <ol class="breadcrumb float-sm-right">
+          <li class="breadcrumb-item"><a href="#">Home</a></li>
+          <li class="breadcrumb-item active">Dashboard v1</li>
+        </ol>
+      </div>
+    </div>
+  </div>
+</div>
+```
+  </div>
+</div>
+
+##### Content
+The content has no change, we only split `<section class="content container-fluid">` in two elements:
+- `<section class="content">`
+- `<div class="container-fluid">`
+
+##### Footer
+The footer has only one little change for the right sided div.
+
+- `<div class="pull-right hidden-xs">` changed to `<div class="float-right d-none d-sm-inline">`
+
+##### Miscellaneous
+Here are some other little css/html changes since v3.0:
+- `.label` renamed to `.badge`
+
+
+##### JavaScript Plugins
+Here are some other little JavaScript changes since v3.0:
+- `data-toggle="*"` renamed to `data-widget="*"` instead of CardWidget items
+  - for CardWidget it's now `data-card-widget="*"`