Create migration guide for v2.2.0 -> v3.0.0

[ZLLentz]

Current Behavior

There are many changes in the next tag. Most (all?) of them are are backwards compatible, but people may get confused and extraneous records may be generated.

Expected Behavior

There should be a clear migration path/upgrade guide.

Context / environment

PR review: #163 (comment)

Tangential: this may be an increase (from 9, if I recall correctly?) from existing projects. We should be careful not to generate extraneous EPICS records in those projects on their next library version bump and subsequent release. Thoughts on where to record this "TODO" item? Individual repo issues?

Maybe every motion repo should get a TODO item for minimizing their states setting?
Actually, what I should do is drop a migration issue on every motion repo that references a gist or something

Suggested Solution

Write and distribute an upgrade guide.