Summary:
When attempting to install PyTorch locally on my Macbook, I had some difficulty running the setup steps and understanding what I was really doing. I've added some clarifications and summarized some debugging steps about `python setup.py develop` to lower the barrier of entrance for new contributors.

I'm seeking a lot of review here since I am not sure if what I wrote is entirely the most useful or accurate. Thank you!

Pull Request resolved: https://github.com/pytorch/pytorch/pull/45903

Reviewed By: albanD

Differential Revision: D24140343

Pulled By: janeyx99

fbshipit-source-id: a5e40d1bc804945ae7db2b95ab18cf7fe169e68a