Update readme for install via pip from repo using github ssh keys

[Chaffelson]

This option should be added to the readme
pip install git+https://github.com/cloudera-labs/cdpy@devel#egg=cdpy

[rjendoubi]

+1. pip install cdpy doesn't work, presumably for the same reason https://pypi.org/project/cdpy doesn't exist.

My only suggestion would be to have the example in the readme show installing the @main branch, not @devel, because you know the people who copy-paste without checking are probably the ones who want main.