Instructions to verify installation

[DanielLenz]

Hi, I have the pleasure to review deepCR for your JOSS submission.

I'll likely have a couple of smaller comments and questions during the next few days, and will raise separate issues to keep your github issues clean.

Could you add a quick paragraph in the README that allows the user to verify the installation, i.e. execute the tests locally? Ideally, you could just grab the matching lines from your travis config (pip install pytest-XYZ; python setup.py test).