sqlalchemy required, but not documented

[mrobbert]

I found that the arbiter service wasn't started on a new install of mine and traced it to the fact that it was trying to load the sqlalchemy module which wasn't installed. I looked again at the pip install that was mentioned in the documentation and setup.py and this module wasn't included there.

[dylngg]

Thanks for letting us know! I've noted that in the README and added a requirements.txt file for Python.