[ci skip] Note in README about Python 2.x

[parente]

Addresses #440

[parente]

@guillaumerosinosky Sorry for the surprise. We try to make it clear that latest is an ever changing set of images in the README, but I understand that in the case of removing Python 2, a note on the google group at least probably would have been a good idea.

This PR adds a note about the last tag containing Python 2.7. I think the recipes wiki page would be the right place for a longer Dockerfile example for how to add Python 2.x back into your own images. Do you agree?

[minrk]

I think the recipes wiki page would be the right place for a longer Dockerfile example for how to add Python 2.x

Yes, that makes sense.