Add a more detailed explanation in Readme of git history and multidev details

[stevector]

The push job as currently written doesn't just push. It copies the state of the code in the GitHub repo and makes a new commit on top of the Pantheon repo. That should be explained in the readme.

The readme should also explain the implications of multidev availability.

[Kerby70]

Would this explain why a persistent Multidev environment branch could have some commits stuck in it that were never merged to the external git master branch thus always showing the one Multidev as having commits ahead?

Is there a method in the orb to always reset a given multidev branch from master before the push?