Iglistkit: Migration guide for 2.x to 3.x breaking changes

[jessesquires]

We'll need to add a new section to the migration guide for the 3.0 release.

https://github.com/Instagram/IGListKit/blob/master/Guides/Migration.md

Maybe we can keep a running list on this thread?

[agisilaos]

Hey @jessesquires! Gonna start diving around to help you with this task! :)

[jessesquires]

Hey @agisilaos ! 💯 Awesome 🎉

So:

• there's one guide example already (1.x to 2.x) -- let's follow that format.
• all changes are in the CHANGELOG

The migration guide should focus on the big changes and how to adopt them. We don't need to discuss small things, like method/class renaming — the CHANGELOG is sufficient for that.

[rnystrom]

Super important are the breaking changes for 3.0.0. I don't think there's anything too crazy in there.

Thanks @agisilaos for taking this on!! This will help us release 3.0 even faster.

[rnystrom]

Also we should detail how to migrate from the old grid layout to the new one. See #559