Document the need for test coverage
diff --git a/docs/contributing.rst b/docs/contributing.rst
index 100f13f..a8010a9 100644
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -28,6 +28,7 @@
* If somehow the tests get into a failing state on ``master`` (such as by a
backwards incompatible release of a dependency) no pull requests may be
merged until this is rectified.
+* All merged patches must have 100% test coverage.
The purpose of these policies is to minimize the chances we merge a change
which jeopardizes our users' security.