Add table of contents in README

Author: ydah

README.md

@@ -7,6 +7,15 @@
 [RSpec](https://rspec.info/)-specific analysis for your projects, as an extension to
 [RuboCop](https://github.com/rubocop/rubocop).
 
+- [Installation](#installation)
+  - [Upgrading to RuboCop RSpec v3.x](#upgrading-to-rubocop-rspec-v3x)
+  - [Upgrading to RuboCop RSpec v2.x](#upgrading-to-rubocop-rspec-v2x)
+- [Usage](#usage)
+- [Documentation](#documentation)
+- [The Cops](#the-cops)
+- [Contributing](#contributing)
+- [License](#license)
+
 ## Installation
 
 Just install the `rubocop-rspec` gem