New doc explaining the different Pantheon GitHub organizations

[stevector]

I'm aware of at least these:

• https://github.com/pantheon-systems
• https://github.com/pantheon-upstreams
• https://github.com/pantheon-se
• https://github.com/orgs/pantheondemo
• https://github.com/pantheon-decoupled-projects
• https://github.com/pantheon-deprecated
• https://github.com/lcache

I think in many cases, the appropriate resolution is shutting down some of these orgs entirely. For instance, I think I was the creator of pantheon-deprecated before GitHub had included "archive" functionality. If first-class archiving existed at the time, we wouldn't have made a separate org.