Fixed #16817 - Added a guide of code coverage to contributing docs.

Thanks Pedro Lima for the draft patch.

.gitignore

@@ -4,3 +4,5 @@
 MANIFEST
 dist/
 docs/_build/
+tests/coverage_html/
+tests/.coverage
\ No newline at end of file

.hgignore

@@ -4,3 +4,5 @@ syntax:glob
 *.pot
 *.py[co]
 docs/_build/
+tests/coverage_html/
+tests/.coverage
\ No newline at end of file

docs/internals/contributing/writing-code/unit-tests.txt

@@ -163,6 +163,26 @@ associated tests will be skipped.
 .. _gettext: http://www.gnu.org/software/gettext/manual/gettext.html
 .. _selenium: http://pypi.python.org/pypi/selenium
 
+Code coverage
+~~~~~~~~~~~~~
+
+Contributors are encouraged to run coverage on the test suite to identify areas
+that need additional tests. The coverage tool installation and use is described
+in :ref:`testing code coverage<topics-testing-code-coverage>`.
+
+To run coverage on the Django test suite using the standard test settings::
+
+    coverage run ./runtests.py --settings=test_sqlite
+
+After running coverage, generate the html report by running::
+
+    coverage html
+
+When running coverage for the Django tests, the included ``.coveragerc``
+settings file  defines ``coverage_html`` as the output directory for the report
+and also excludes several directories not relevant to the results
+(test code or external code included in Django).
+
 .. _contrib-apps:
 
 Contrib apps

docs/topics/testing.txt

@@ -589,6 +589,8 @@ to a faster hashing algorithm::
 Don't forget to also include in :setting:`PASSWORD_HASHERS` any hashing
 algorithm used in fixtures, if any.
 
+.. _topics-testing-code-coverage:
+
 Integration with coverage.py
 ----------------------------
 

tests/.coveragerc

+[run]
+omit = runtests,test_sqlite,regressiontests*,modeltests*,*/django/contrib/*/tests*,*/django/utils/unittest*,*/django/utils/simplejson*,*/django/utils/importlib.py,*/django/test/_doctest.py,*/django/core/servers/fastcgi.py,*/django/utils/autoreload.py,*/django/utils/dictconfig.py
+
+[html]
+directory = coverage_html