Document python dependency name

[Kashyap Vedurmudi]

We should add some documentation to the python-runtime-cnb that states what dependencies are provided and how to require them.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/172504453

The labels on this github issue will be updated when the story is started.

[Tim Hitchener]

@kvedurmu closing this as I think #8 has resolved it.