README: add Circle CI badge

Author: tarleb

README.md

@@ -1,8 +1,10 @@
 # Lua Filters
 
 [![Travis build status][Travis badge]](https://travis-ci.org/pandoc/lua-filters)
+[![Circle CI build status][Circle CI badge]](https://circleci.com/gh/pandoc/lua-filters)
 
 [Travis badge]: https://img.shields.io/travis/pandoc/lua-filters.svg?logo=travis
+[Circle CI badge]: https://img.shields.io/circleci/build/gh/pandoc/lua-filters?logo=circleci
 
 This repository collects Lua filters for pandoc.