[DOCS] Add a section on testing to CONTRIBUTING.md (#767)

This change mirrors exactly what we already have at our sister project.

Author: oliverklee

CONTRIBUTING.md

@@ -71,3 +71,14 @@ You can then run the following command:
 phive install
 ```
 
+## Unit-test your changes
+
+Please cover all changes with unit tests and make sure that your code does not
+break any existing tests. We will only merge pull requests that include full
+code coverage of the fixed bugs and the new features.
+
+To run the existing PHPUnit tests, run this command:
+
+```bash
+composer ci:tests:unit
+```