update README, add coveralls badge

Author: tonerdo

README.md

@@ -1,4 +1,5 @@
 [![Build Status](https://www.travis-ci.org/tonerdo/coverlet.svg?branch=master)](https://www.travis-ci.org/tonerdo/coverlet)
 [![Build status](https://ci.appveyor.com/api/projects/status/6rdf00wufospr4r8?svg=true)](https://ci.appveyor.com/project/tonerdo/coverlet)
+[![Coverage Status](https://coveralls.io/repos/github/tonerdo/coverlet/badge.svg?branch=master)](https://coveralls.io/github/tonerdo/coverlet?branch=master)
 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](LICENSE)
 # coverlet