Add contributing doc and badge

Author: AdamDonna

CONTRIBUTING.rst

@@ -0,0 +1,5 @@
+.. image:: https://jazzband.co/static/img/jazzband.svg
+   :target: https://jazzband.co/
+   :alt: Jazzband
+
+This is a `Jazzband <https://jazzband.co>`_ project. By contributing you agree to abide by the `Contributor Code of Conduct <https://jazzband.co/about/conduct>`_ and follow the `guidelines <https://jazzband.co/about/guidelines>`_.

README.rst

@@ -6,6 +6,9 @@
     :target: https://codecov.io/github/django-polymorphic/django-polymorphic?branch=master
 .. image:: https://readthedocs.org/projects/django-polymorphic/badge/?version=stable
     :target: https://django-polymorphic.readthedocs.io/en/stable/
+.. image:: https://jazzband.co/static/img/badge.svg
+   :target: https://jazzband.co/
+   :alt: Jazzband
 
 Polymorphic Models for Django
 =============================