Add badges in README (#2432)

README.md

@@ -1,6 +1,11 @@
 CSL Style Repository
 ====================
 
+[![Build Status](https://travis-ci.org/citation-style-language/styles.svg?branch=master)](https://travis-ci.org/citation-style-language/styles)
+[![license](https://img.shields.io/badge/license-CC BY SA 3.0-blue.svg)](https://github.com/citation-style-language/styles#licensing)
+[![GitHub contributors](https://img.shields.io/github/contributors/citation-style-language/styles.svg)](https://github.com/citation-style-language/styles/graphs/contributors)
+[![GitHub closed pull requests](https://img.shields.io/github/issues-pr-closed/citation-style-language/styles.svg)](https://github.com/citation-style-language/styles/pulls?q=is%3Apr+is%3Aclosed)
+
 [github.com/citation-style-language/styles](https://github.com/citation-style-language/styles) is the official repository for Citation Style Language (CSL) styles and is maintained by CSL project members.
 For more information, check out [CitationStyles.org](http://citationstyles.org/) and the [repository wiki](https://github.com/citation-style-language/styles/wiki).