Add info about contributing to docs to CONTRIBUTING.md

Encourage more people to contribute to Rails docs by mentioning it in the contribution guide.

CONTRIBUTING.md

@@ -4,6 +4,8 @@ Ruby on Rails is a volunteer effort. We encourage you to pitch in. [Join the tea
 
 * If you want to submit a patch, please read the [Contributing to Ruby on Rails](http://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html) guide.
 
+* If you want to contribute to Rails documentation, please read the [Contributing to the Rails Documentation](http://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html#contributing-to-the-rails-documentation) section of the aforementioned guide.
+
 *We only accept bug reports and pull requests in GitHub*.
 
 * If you have a question about how to use Ruby on Rails, please [ask the rubyonrails-talk mailing list](https://groups.google.com/forum/?fromgroups#!forum/rubyonrails-talk).