This adds more details on the PYTHON[3]_PKG_SETUP_* variables, the
filespec format, and the build/install processes in general.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

It was a semi-popular request a while back that I put off for a while.
Here's a quick doc.
I'll update it later as things change and I don't forget.
It also lets other people update if something is wrong.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>