Merge pull request django-cms#3508 from yakky/feature/doc_coverage

Declare new testing policy

Author: yakky

CONTRIBUTING.rst

@@ -109,7 +109,10 @@ And at any point in that process, you can add: *discuss discuss discuss*,
 because it's always useful for everyone to pass ideas around and look at thngs
 together.
 
-:ref:`testing` is really important.
+:ref:`testing` is really important: a pull request that lowers our testing
+coverage will only be accepted with a very good reason; bug-fixing patches
+**must** demonstrate the bug with a test to avoid regressions and to check
+that the fix works.
 
 We have an IRC channel, our `django-cms-developers`_ email list,
 and of course the code reviews mechanism on GitHub - do use them.