Fixes #509 - mention [ci skip] in docs.

Author: Mike Taylor

CONTRIBUTING.md

@@ -318,6 +318,8 @@ You can run the Python unit tests from the project root with the `nosetests` com
 
 Running functional tests is a bit more involved (see the next section).
 
+Tests are also run automatically on [Travis](https://travis-ci.org/webcompat/webcompat.com) for each commit. If you would like to skip running tests for a given commit, you can use use the magical `[ci skip]` string in your commit message. See the [Travis docs](http://docs.travis-ci.com/user/how-to-skip-a-build/#Not-All-Commits-Need-CI-Builds) for more info.
+
 ### Functional Tests
 
 We use [Intern](http://theintern.io/) to run functional tests.