README.rst: Add status badges for PyPI release and Python versions

The first badge will work fine right away while the second badge will
show "not found" until a release is made after merging this branch:
!718.

README.rst

@@ -13,6 +13,9 @@ About
 .. image:: https://gitlab.com/BuildStream/buildstream/badges/master/coverage.svg?job=coverage
    :target: https://gitlab.com/BuildStream/buildstream/commits/master
 
+.. image:: https://img.shields.io/pypi/v/BuildStream.svg
+   :target: https://pypi.org/project/BuildStream
+
 
 What is BuildStream?
 ====================