doc(README): Add monthly download stats badge

In addition of the total download, provide an idea of the relative current usage by also report the monthly download stats.

Co-authored-by: Henry Schreiner <[email protected]>

Author: jcfr

README.rst

@@ -23,6 +23,8 @@ Latest Release
   +======================================================================+===========================================================================+
   | .. image:: https://img.shields.io/pypi/v/cmake.svg                   | .. image:: https://static.pepy.tech/badge/cmake                           |
   |     :target: https://pypi.python.org/pypi/cmake                      |     :target: https://pypi.python.org/pypi/cmake                           |
+  |                                                                      | .. image:: https://img.shields.io/pypi/dm/cmake                           |
+  |                                                                      |     :target: https://pypi.python.org/pypi/cmake                           |
   +----------------------------------------------------------------------+---------------------------------------------------------------------------+
 
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