Add Coverage badge

Author: mhallin

README.md

@@ -4,6 +4,7 @@
 
 [![build status](https://img.shields.io/travis/mhallin/juniper.svg?maxAge=2592000&style=flat-square)](https://travis-ci.org/mhallin/juniper)
 [![Crates.io](https://img.shields.io/crates/v/juniper.svg?maxAge=2592000&style=flat-square)](https://crates.io/crates/juniper)
+[![Coveralls](https://img.shields.io/coveralls/jekyll/jekyll.svg?maxAge=2592000&style=flat-square)](https://coveralls.io/github/mhallin/juniper)
 
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