adding .coveragerc and docs about how to use it

.coveragerc

@@ -0,0 +1,13 @@
+[run]
+source = normandy
+branch = True
+data_file = /tmp/.coverage
+
+[report]
+omit =
+    tests/*
+    normandy/settings.py
+    normandy/wsgi.py
+
+[xml]
+output = /tmp/coverage.xml

docs/dev/coverage.rst

@@ -0,0 +1,35 @@
+=============
+Test Coverage
+=============
+
+We do not include test coverage as a metric or a requirement in CI but test coverage as a
+local tool is useful to make sure your code does test all the things you care about.
+
+To use it, install ``coverage`` in your virtualenv env.
+
+.. code-block:: bash
+
+    pip install coverage
+
+Now run the tests with coverage like this:
+
+.. code-block:: bash
+
+    coverage run -m pytest normandy
+
+To generate a report, run:
+
+.. code-block:: bash
+
+    coverage html --skip-covered
+
+This will create the HTML report in the ``./htmlcov/`` directory.
+
+.. code-block:: bash
+
+    open htmlcov/index.html
+
+.. note::
+
+    The ``--skip-covered`` flag means it doesn't create a HTML report for files with 100%
+    test coverage.

docs/dev/index.rst

@@ -14,6 +14,7 @@ of the Normandy client.
    workflow
    api-tests
    code-style
+   coverage
    feature-experiments
    troubleshooting
    ci