Add CircleCI build status badge

README.md

@@ -1,3 +1,4 @@
+[![Build Status](https://circleci.com/gh/diasurgical/devilution.svg?style=svg)](https://circleci.com/gh/diasurgical/devilution)
 [![Build Status](https://www.travis-ci.com/diasurgical/devilution.svg?branch=master)](https://www.travis-ci.com/diasurgical/devilution)
 [![Build status](https://ci.appveyor.com/api/projects/status/ssk0xjhoka1uu940?svg=true)](https://ci.appveyor.com/project/galaxyhaxz/devilution)
 [![Build status](https://img.shields.io/github/downloads/diasurgical/devilution/total.svg)](https://github.com/diasurgical/devilution/releases)