Add PyPI badge on README

Author: seratch

README.rst

@@ -1,8 +1,10 @@
 Slack Events API adapter for Python
 ===================================
 
+.. image:: https://badge.fury.io/py/slackeventsapi.svg
+  :target: https://pypi.org/project/slackeventsapi/
 .. image:: https://travis-ci.org/slackapi/python-slack-events-api.svg?branch=master
-    :target: https://travis-ci.org/slackapi/python-slack-events-api
+  :target: https://travis-ci.org/slackapi/python-slack-events-api
 .. image:: https://codecov.io/gh/slackapi/python-slack-events-api/branch/master/graph/badge.svg
   :target: https://codecov.io/gh/slackapi/python-slack-events-api