Make contributors aware of the pydocstyle requirements. (#3805)

Make contributors aware of the new pydocstyle test.

CONTRIBUTING.rst

@@ -60,7 +60,8 @@ Here are some guidelines about how you can contribute to Nikola:
 * If you are fixing an issue, **include the issue number in commit** and/or pull
   request text (eg. ``fix #1234``) so the issue `is automatically closed
   <https://help.github.com/articles/closing-issues-via-commit-messages/>`_.
-* Run ``flake8 nikola tests`` for **style consistency**.
+* Run ``flake8 nikola tests`` and ``pydocstyle --count --match-dir='(?!^\\.)(?!data).*' nikola/``
+  for **style consistency**.
 * Ensure your Git name and e-mail are set correctly (they will be public)
   and `added to GitHub <https://github.com/settings/emails>`_
 * **Try writing some tests** if possible — again, following existing tests is

requirements-tests.txt

@@ -5,3 +5,4 @@ pytest-cov>=2.6.1
 feedparser>=6.0.10
 freezegun>=0.3.10
 flake8>=3.7.7
+pydocstyle>=6.3.0