Fixed #18971 -- added root level CONTRIBUTING doc

CONTRIBUTING.rst

@@ -0,0 +1,16 @@
+======================
+Contributing to Django
+======================
+
+As an open source project, Django welcomes contributions of many forms.
+
+Examples of contributions include:
+
+* Code patches
+* Documentation improvements
+* Bug reports and patch reviews
+
+Extensive contribution guidelines are available in the repository at
+``docs/internals/contributing/``, or online at:
+
+https://docs.djangoproject.com/en/dev/internals/contributing/