Add installation instructions to README.md

[SkotBotCambo]

I 100% appreciate the care that was given to make sure that this package is pip installable from a well organized GH repo, but I was surprised to find that I would have to find the egg name from setup.py. It was a minor inconvence, but just adding an "Installation" section to the readme with the following line will probably be very helpful for others too. Cheers!

pipenv install git+https://github.com/christopherjenness/DBCV.git#egg=DBCV