Add PyPI downloads badge to README

Author: peterjc

README.rst

@@ -7,6 +7,9 @@ flake8-rst-docstrings
 .. image:: https://img.shields.io/travis/peterjc/flake8-rst-docstrings/master.svg
    :alt: Testing with TravisCI
    :target: https://travis-ci.org/peterjc/flake8-rst-docstrings/branches
+.. image:: https://img.shields.io/pypi/dm/flake8-rst-docstrings.svg
+   :alt: PyPI downloads
+   :target: https://pypistats.org/packages/flake8-rst-docstrings
 .. image:: https://landscape.io/github/peterjc/flake8-rst-docstrings/master/landscape.svg?style=flat
    :alt: Landscape Code Metrics
    :target: https://landscape.io/github/peterjc/flake8-rst-docstrings/