Add coverage badge

README.md

@@ -2,7 +2,8 @@
 
 [![npm version][version-badge]][version-url]
 [![build status][build-badge]][build-url]
-[![greenkeeper status][greenkeeper-badge]][greenkeeper-url]
+[![coverage status][coverage-badge]][coverage-url]
+[![greenkeeper][greenkeeper-badge]][greenkeeper-url]
 [![downloads][downloads-badge]][downloads-url]
 
 > disallow features that aren't supported by your target browser audience
@@ -104,3 +105,5 @@ Last 2 versions
 [downloads-url]: https://www.npmjs.com/package/stylelint-no-unsupported-browser-features
 [version-badge]: https://img.shields.io/npm/v/stylelint-no-unsupported-browser-features.svg
 [version-url]: https://www.npmjs.com/package/stylelint-no-unsupported-browser-features
+[coverage-badge]: https://coveralls.io/repos/github/ismay/stylelint-no-unsupported-browser-features/badge.svg?branch=master
+[coverage-url]: https://coveralls.io/github/ismay/stylelint-no-unsupported-browser-features?branch=master