Add link to Python Package Index

Signed-off-by: Michal Čihař <michal@cihar.com>

README.rst

@@ -29,6 +29,9 @@ Weblate
     :alt: Bountysource
     :target: https://www.bountysource.com/trackers/253393-weblate?utm_source=253393&utm_medium=shield&utm_campaign=TRACKER_BADGE
 
+.. image:: https://img.shields.io/pypi/v/Weblate.svg
+    :target: https://pypi.python.org/pypi/Weblate
+
 Weblate is a free web-based translation tool with tight version control
 integration. It features simple and clean user interface, propagation of
 translations across components within a project, quality checks and automatic