add link to Coveralls in README.md

Author: stefankoegl

README.md

@@ -13,6 +13,7 @@ See Sourcecode for Examples
 * Documentation: https://python-json-patch.readthedocs.org/
 * PyPI: https://pypi.python.org/pypi/jsonpatch
 * Travis-CI: https://travis-ci.org/stefankoegl/python-json-patch
+* Coveralls: https://coveralls.io/r/stefankoegl/python-json-patch
 
 Running external tests
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