Add link to testing README in CONTRIBUTING.md

Author: nat-n

CONTRIBUTING.md

@@ -15,9 +15,9 @@ What can you do?
     - File a bug (please check its not a duplicate)
     - Propose an enhancement
 - :white_check_mark: Create a PR:
-    - Creating a failing test-case to make bug-fixing easier
+    - [Creating a failing test-case](https://github.com/danielgtaylor/python-betterproto/blob/master/betterproto/tests/README.md) to make bug-fixing easier
     - Fix any of the open issues
         - [Good first issues](https://github.com/danielgtaylor/python-betterproto/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22)
         - [Issues with tests](https://github.com/danielgtaylor/python-betterproto/issues?q=is%3Aissue+is%3Aopen+label%3A%22has+test%22)
     - New bugfix or idea
-        - If you'd like to discuss your idea first, join us on Slack!
+        - If you'd like to discuss your idea first, join us on Slack!