Add coveralls badge

Author: ghalse

README.md

@@ -1,6 +1,7 @@
 fticks:Fticks
 =============
 [![Build Status](https://travis-ci.org/simplesamlphp/simplesamlphp-module-fticks.svg?branch=master)](https://travis-ci.org/simplesamlphp/simplesamlphp-module-fticks)
+[![Coverage Status](https://coveralls.io/repos/github/simplesamlphp/simplesamlphp-module-fticks/badge.svg?branch=master)](https://coveralls.io/github/simplesamlphp/simplesamlphp-module-fticks?branch=master)
 
 Log statistics in the [F-ticks federation log format](https://wiki.geant.org/display/gn42jra3/F-ticks+standard).