Migration guides

[elifoster]

Some general migration guides would be good. Since many people are still on 1.7, I think it would be good to start there:

• 1.7 -> 1.8
• 1.8 -> 1.9
• 1.9 -> 1.10
• 1.10 -> 1.11
• 1.11 -> 1.12

and then any future versions as well.