Add PyPI badges

Author: Leonidas Poulopoulos

README.md

@@ -1,4 +1,7 @@
 [![Build Status](https://travis-ci.org/leopoul/ncclient.svg?branch=master)](https://travis-ci.org/leopoul/ncclient)
+[![PyPi version](https://pypip.in/v/ncclient/badge.png)](https://crate.io/packages/ncclient/)
+[![PyPi downloads](https://pypip.in/d/ncclient/badge.png)](https://crate.io/packages/ncclient/)
+[![Documentation Status](https://readthedocs.org/projects/ncclient/badge/?version=latest)](https://readthedocs.org/projects/ncclient/?badge=latest)
 
 ncclient: Python library for NETCONF clients
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