Adding section for private repos

[thadc23]

This new section talks about how to setup a flag for pip to point to an internal dependency repository and incorporates the certificate workaround defined here

I think surfacing this information in docs is preferable to having it buried in an issue on the buildpack. It also fills the gap between online and offline use.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

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

[cfdreddbot]

Hey thadc23!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[animatedmax]

Thank you, @thadc23