Add RTD badge

Author: SylvainCorlay

README.md

@@ -1,6 +1,7 @@
 # Scipy Trait Types
 
 [![Build Status](https://travis-ci.org/jupyter-incubator/traittypes.svg?branch=master)](https://travis-ci.org/jupyter-incubator/traittypes)
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 Trait types for NumPy, SciPy and friends