Expanding contribution documentation (#1883)

- README updated with link to CONTRIBUTING.rst
- CONTRIBUTING.rst has instructions on running code coverage

CONTRIBUTING.rst

@@ -68,3 +68,22 @@ of ``pytest``. You can install it with::
 
 The ``tox`` command will then run all tests against multiple combinations
 Python versions and dependency versions.
+
+Running test coverage
+---------------------
+Generating a report of lines that do not have unit test coverage can indicate where
+to start contributing.  ``pytest`` integrates with ``coverage.py``, using the ``pytest-cov``
+plugin.  This assumes you have already run the testsuite (see previous section)::
+
+    pip install pytest-cov
+
+After this has been installed, you can output a report to the command line using this command::
+
+    py.test --cov=flask tests/
+
+Generate a HTML report can be done using this command::
+
+    py.test --cov-report html --cov=flask tests/
+
+Full docs on ``coverage.py`` are here: https://coverage.readthedocs.io
+

README

@@ -37,6 +37,8 @@
 
         $ py.test
 
+      Details on contributing can be found in CONTRIBUTING.rst
+
     ~ Where can I get help?
 
       Either use the #pocoo IRC channel on irc.freenode.net or