README: add CircleCI's Status badge

README.md

@@ -2,6 +2,7 @@
 
 [![Build status](https://ci.appveyor.com/api/projects/status/399d4hcw9yy7hg2y?svg=true)](https://ci.appveyor.com/project/STJr/srb2)
 [![Build status](https://travis-ci.org/STJr/SRB2.svg?branch=master)](https://travis-ci.org/STJr/SRB2)
+[![CircleCI](https://circleci.com/gh/STJr/SRB2/tree/master.svg?style=svg)](https://circleci.com/gh/STJr/SRB2/tree/master)
 
 [Sonic Robo Blast 2](https://srb2.org/) is a 3D Sonic the Hedgehog fangame based on a modified version of [Doom Legacy](http://doomlegacy.sourceforge.net/).