Intro text for "supporting multiple Python versions".

This one should come before the others. In my efforts to make review of these PRs easy (by splitting up the changes), I may have messed things up. Hopefully there are no merge conflicts.

This PR is some intro text that appears first in the "multiple python versions" section.

The order should be:

- pymultiverintrotext
- greghewgillref
- continuousintegration
- singlesourcepython
- whatwhichpython