Add information about how to run tests

This is something that took me a few minutes to find, so let's make it easier for new people to know how to run a specific test, as that's very commonly used when preparing a PR. 

As there is a lot of potential small changes in this PR, please feel free to directly apply any changes and merge from there.

CONTRIBUTING.md

@@ -75,6 +75,10 @@ Code/comments should adhere to the following rules:
 * All changes require test coverage to ensure it does not break during refactor
   work. (There are a few exceptions to this, which can be recognised by there
   not being any coverage for similar code.)
+  * CocoaPods uses [bacon](https://github.com/chneukirchen/bacon) as a test runner.
+    To run all tests, use `bundle exec rake spec` in the root of the 
+    [Rainforest repo](https://github.com/CocoaPods/Rainforest). If you want to run
+    a specific test instead, use `bundle exec bacon spec/[folder]/[name]_spec.rb`
 * All enhancements and bug fixes need to be documented in the CHANGELOG.
 * When writing comments, use properly constructed sentences, including
   punctuation.