Add format instruction to contribution guidelines

Author: meatballs

docs/tutorials/contributing/guidelines.rst

@@ -11,15 +11,18 @@ The project follows the following guidelines:
 2. Try as best as possible to follow `PEP8
    <https://www.python.org/dev/peps/pep-0008/>`_ which includes **using
    descriptive variable names**.
-3. Commits: Please try to use commit messages that give a meaningful history
+3. Code Format: Use the `Black formatter <https://github.com/ambv/black>`_ to format
+    all code  and the `isort utility <https://github.com/timothycrosley/isort>`_ to
+    sort import statements.
+4. Commits: Please try to use commit messages that give a meaningful history
    for anyone using git's log features. Try to use messages that complete sentence,
    "This commit will..." There is some excellent guidance on the subject
    from `Chris Beams <https://chris.beams.io/posts/git-commit/>`_
-4. Testing: the project uses the `unittest
+5. Testing: the project uses the `unittest
    <https://docs.python.org/2/library/unittest.html>`_ library and has a nice
    testing suite that makes some things very easy to write tests for. Please try
    to increase the test coverage on pull requests.
-5. Merging pull-requests: We require two of the (currently three) core-team
+6. Merging pull-requests: We require two of the (currently three) core-team
    maintainers to merge. Opening a PR for early
    feedback or to check test coverage is OK, just indicate that the PR is not ready
    to merge (and update when it is).