Document the process for deprecating an API in React

[flarnie]

Split out from #9398

• what is the process for making that decision?
• how do we communicate it before and during the release?
• how do we support folks updating their syntax?
• what should be included in a deprecation warning?

[gaearon]

Do we plan to do it? The current ad hoc process works. Who will read this and where?
If you don't feel strongly we can close it and revisit next time we deprecate something.

[flarnie]

It was something that commenters had flagged as confusing on one of our release/checklist items, but since joining the team I observed that we have an "ad hoc" process that works, and it isn't something that seems useful to document. Let's close this for now - thanks!