contrib: Update mention of tests

* Include a link to the new created test directory.

Author: kylemanna

CONTRIBUTING.md

@@ -16,9 +16,11 @@ Please add new documentation in the `docs` folder for any new features.  Pull re
 
 If you have a special feature, you're likely to try but it will likely be rejected if not too many people seem interested.
 
-## Tests (TODO)
+## Tests
 
-In an effort to not repeat bugs (and break less popular features), unit tests are run on [Travis CI](https://travis-ci.org/kylemanna/docker-bitcoind).  The goal of the tests are to be simple and to be placed in the `tests` directory where it will be automatically run.  Review existing tests for an example.
+In an effort to not repeat bugs (and break less popular features), unit tests are run on [Travis CI](https://travis-ci.org/kylemanna/docker-bitcoind).  The goal of the tests are to be simple and to be placed in the `test` directory where it will be automatically run.
+
+See [test directory](https://github.com/kylemanna/docker-bitcoind/tree/master/test) for details.
 
 ## Style