add readthedocs badge

Author: robphoenix

README.md

@@ -6,6 +6,7 @@
 [![PyPI](https://img.shields.io/pypi/pyversions/diffios.svg?style=flat-square)](https://pypi.python.org/pypi/diffios)
 [![PyPI](https://img.shields.io/pypi/status/diffios.svg?style=flat-square)](ttps://pypi.python.org/pypi/diffios)
 [![Coveralls](https://img.shields.io/coveralls/robphoenix/diffios.svg?style=flat-square)](https://coveralls.io/github/robphoenix/diffios?branch=master)
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 diffios is a Python library which provides a way to compare Cisco IOS configurations
 against a baseline template, and generate an output detailing the differences