Add brief description and caveats of project - virtualenv mapping

[floatingpurr]

The issue

It is not immediately clear how pipenv knows the virtualenv for a project (See #796)

The fix

Docs extension

[uranusjr]

Thanks for putting this together! I don’t feel this should be on the front page—it already has too many things (I’ve been thinking if it’s possible to removing things from it, actually). How about putting this in either Pipenv & Virtual Environments or Advanced Usage of Pipenv?

[floatingpurr]

Hey @uranusjr, thank you for your suggestion. I did it.