Extended readme for testruns

Hey guys

I'm just about to dive in this whole phpunit thing and it took me a little until I finally got it to work so I thought I just write down my findings in the README, hoping that it will save future user's time when trying to run the tests :-)

I hope it's all fine (also with the composer versioning...) and if not I would appreciate your feedback so I can modify the pull request and learn from you guys :-)

Thanks!