Add index to readme [ci skip]

README.md

@@ -5,6 +5,50 @@
 [![Dependency Status](https://gemnasium.com/aasm/aasm.svg)](https://gemnasium.com/aasm/aasm)
 [![Code Climate](https://codeclimate.com/github/aasm/aasm/badges/gpa.svg)](https://codeclimate.com/github/aasm/aasm)
 
+## Index
+- [Upgrade from version 3 to 4](#upgrade-from-version-3-to-4)
+- [Usage](#usage)
+  - [Callbacks](#callbacks)
+    - [Lifecycle](#lifecycle)
+    - [The current event triggered](#the-current-event-triggered)
+  - [Guards](#guards)
+  - [Transitions](#transitions)
+  - [Multiple state machines per class](#multiple-state-machines-per-class)
+    - [Handling naming conflicts between multiple state machines](#handling-naming-conflicts-between-multiple-state-machines)
+    - [Binding event](#binding-event)
+  - [Auto-generated Status Constants](#auto-generated-status-constants)
+  - [Extending AASM](#extending-aasm)
+  - [ActiveRecord](#activerecord)
+  - [Bang events](#bang-events)
+  - [ActiveRecord enums](#activerecord-enums)
+  - [Sequel](#sequel)
+  - [Dynamoid](#dynamoid)
+  - [Mongoid](#mongoid)
+  - [Redis](#redis)
+  - [Automatic Scopes](#automatic-scopes)
+  - [Transaction support](#transaction-support)
+  - [Pessimistic Locking](#pessimistic-locking)
+  - [Column name & migration](#column-name--migration)
+  - [Inspection](#inspection)
+  - [Warning output](#warning-output)
+  - [RubyMotion support](#rubymotion-support)
+  - [Testing](#testing)
+    - [RSpec](#rspec)
+    - [Minitest](#minitest)
+      - [Assertions](#assertions)
+      - [Expectations](#expectations)
+ - [Installation](#installation)
+   - [Manually from RubyGems.org](#manually-from-rubygemsorg)
+   - [Bundler](#or-if-you-are-using-bundler)
+   - [Building your own gems](#building-your-own-gems)
+  - [Generators](#generators)
+  - [Latest changes](#latest-changes)
+  - [Questions?](#questions)
+  - [Maintainers](#maintainers)
+- [Contributing](CONTRIBUTING.md)
+- [Warranty](#warranty)
+- [License](#license)
+
 This package contains AASM, a library for adding finite state machines to Ruby classes.
 
 AASM started as the *acts_as_state_machine* plugin but has evolved into a more generic library