Advertise 'tdd' branch as a base for broken unit tests

Author: ianfixes

CONTRIBUTING.md

@@ -3,9 +3,11 @@
 ArduinoCI uses a very standard GitHub workflow.
 
 1. Fork the repository on github
-2. Make your desired changes
+2. Make your desired changes on top of the latest `master` branch
 3. Push to your personal fork
 4. Open a pull request
+    * If you are submitting code, use `master` as the base branch
+    * If you are submitting broken unit tests (illustrating a bug that should be fixed), use `tdd` as the base branch.
 
 Pull requests will trigger a Travis CI job.  The following two commands will be expected to pass (so you may want to run them locally before opening the pull request):