Added PyPI downloads badge

Author: seanlaw

README.rst

@@ -1,12 +1,15 @@
 .. image:: https://img.shields.io/pypi/v/stumpy.svg
  :target: https://pypi.org/project/stumpy/
  :alt: PyPI Version
+.. image:: https://pepy.tech/badge/stumpy
+ :target: https://pepy.tech/project/stumpy
+ :alt: PyPI Downloads
 .. image:: https://anaconda.org/conda-forge/stumpy/badges/version.svg
  :target: https://anaconda.org/conda-forge/stumpy
  :alt: Conda-forge Version
 .. image:: https://anaconda.org/conda-forge/stumpy/badges/downloads.svg
  :target: https://anaconda.org/conda-forge/stumpy
- :alt: Conda-forge downloads
+ :alt: Conda-forge Downloads
 .. image:: https://img.shields.io/pypi/l/stumpy.svg
  :target: https://github.com/TDAmeritrade/stumpy/blob/master/LICENSE.txt
  :alt: License