Document testing and improve status checks section.

README.md

@@ -9,12 +9,25 @@ for production use.**
 **One place for service teams to find styles, components and patterns for
 designing government services.**
 
-## Contribution
+## Running tests
 
-### Automated Checks
+We are using [RSpec][rspec] for our unit and integration tests. You can run the
+tests from command line by running:
 
-When changes are pushed to GitHub [Travis][travis] will attempt to run
-the `middleman build` command to ensure that the site can still be generated
-correctly.
+```
+bundle exec rspec
+```
+
+[rspec]: https://relishapp.com/rspec  
+
+## Automated Checks
+
+When changes are pushed to GitHub [Travis][travis] will:
+
+- run the tests
+- run the `middleman build` command to ensure that the site can be generated
+
+If any of these fail, this will be reported in the GitHub status checks
+interface.
 
 [travis]: https://travis-ci.org/alphagov/govuk-design-system