add readthedocs badge
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py3status
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-|travis|
+|travis| |readthedocs|
.. |travis| image:: https://travis-ci.org/ultrabug/py3status.svg?branch=master
+.. |readthedocs| image:: https://readthedocs.org/projects/py3status/badge/?version=latest
**py3status** is an extensible i3status wrapper written in python.
