Add some documents

This PR adds some documents with reference to RuboCop Performance.
rubocop/rubocop-performance#29

Author: koic

README.md

@@ -1,5 +1,73 @@
-[![CircleCI Status](https://circleci.com/gh/rubocop-hq/rubocop-rails.svg?style=svg)](https://circleci.com/gh/rubocop-hq/rubocop-rails)
-
 # RuboCop Rails
 
-A RuboCop extension focused on enforcing Rails best practices and coding conventions.
+[![CircleCI](https://circleci.com/gh/rubocop-hq/rubocop-rails.svg?style=svg)](https://circleci.com/gh/rubocop-hq/rubocop-rails)
+
+A [RuboCop](https://github.com/rubocop-hq/rubocop) extension focused on enforcing Rails best practices and coding conventions.
+
+## Installation
+
+Just install the `rubocop-rails` gem
+
+```sh
+gem install rubocop-rails
+```
+
+or if you use bundler put this in your `Gemfile`
+
+```ruby
+gem 'rubocop-rails'
+```
+
+## Usage
+
+You need to tell RuboCop to load the Rails extension. There are three
+ways to do this:
+
+### RuboCop configuration file
+
+Put this into your `.rubocop.yml`.
+
+```yaml
+require: rubocop-rails
+```
+
+Now you can run `rubocop` and it will automatically load the RuboCop Rails
+cops together with the standard cops.
+
+### Command line
+
+```sh
+rubocop --require rubocop-rails
+```
+
+### Rake task
+
+```ruby
+RuboCop::RakeTask.new do |task|
+  task.requires << 'rubocop-rails'
+end
+```
+
+## The Cops
+
+All cops are located under
+[`lib/rubocop/cop/rails`](lib/rubocop/cop/rails), and contain
+examples/documentation.
+
+In your `.rubocop.yml`, you may treat the Rails cops just like any other
+cop. For example:
+
+```yaml
+Rails/FindBy:
+  Exclude:
+    - lib/example.rb
+```
+
+## Contributing
+
+Checkout the [contribution guidelines](CONTRIBUTING.md).
+
+## License
+
+`rubocop-rails` is MIT licensed. [See the accompanying file](LICENSE.txt) for
+the full text.

manual/index.md

@@ -0,0 +1 @@
+A [RuboCop](https://github.com/rubocop-hq/rubocop) extension focused on enforcing Rails best practices and coding conventions.

manual/installation.md

@@ -0,0 +1,11 @@
+Just install the `rubocop-rails` gem
+
+```sh
+gem install rubocop-rails
+```
+
+or if you use bundler put this in your `Gemfile`
+
+```ruby
+gem 'rubocop-rails'
+```

manual/usage.md

@@ -0,0 +1,27 @@
+You need to tell RuboCop to load the Rails extension. There are three
+ways to do this:
+
+### RuboCop configuration file
+
+Put this into your `.rubocop.yml`.
+
+```yaml
+require: rubocop-rails
+```
+
+Now you can run `rubocop` and it will automatically load the RuboCop Rails
+cops together with the standard cops.
+
+### Command line
+
+```sh
+rubocop --require rubocop-rails
+```
+
+### Rake task
+
+```ruby
+RuboCop::RakeTask.new do |task|
+  task.requires << 'rubocop-rails'
+end
+```