Add a migration guide from FOSRest

[dunglas]

FOSRestBundle is now abandoned. The main authors of the bundle recommend to use API Platform.
We should add a migration guide for people coming from FOSRest. Basically, it should map FOSRest features with the similar features in API Platform.

Idea from @Deuchnord.

[Deuchnord]

I'd like to take this one :)