Add "Typing Guidance for Python Libraries" document (#889)

By Eric Traut, source: https://raw.githubusercontent.com/microsoft/pyright/main/docs/typed-libraries.md

See srittau/peps#94 for background.

This is a pristine copy of the original document. It needs to be
converted into RestructuredText and linked from the index as a next
step.

Author: srittau