code coverage badge added

README.md

 [![build status](https://git-r3lab.uni.lu/piotr.gawron/minerva/badges/master/build.svg)](https://git-r3lab.uni.lu/piotr.gawron/minerva/commits/master)
+[![coverage report](https://git-r3lab.uni.lu/minerva/core/badges/master/coverage.svg)](https://git-r3lab.uni.lu/minerva/core/commits/master)
 
 # For users
 Documentation for minerva can be found here: https://minerva.pages.uni.lu/doc/