[readme] Add buildbot badge

README.md

@@ -38,6 +38,7 @@ See the file [LICENSE.md](LICENSE.md) for full copying permissions.
 
 Automated Testing
 ==================
+[![buildbot badge](https://git.iws.uni-stuttgart.de/buildbot/badges/dumux-master-dune-release26-gcc.svg)](https://git.iws.uni-stuttgart.de/buildbot/#/builders)
 
 DuMuX features many tests (some unit tests and test problems) that can
 be run manually. We have experimental support for automated testing with buildbot.