Add docs for migrating from AMS.

[beauby]

Identified gotchas:

• attribute :foo, key: :bar: in AMS, this creates a bar attribute in the JSON output. In jsonapi-rb, this becomes attribute(:bar) { @object.foo }.
• some other stuff I haven't identified yet.

[richmolj]

As AMS has a default jsonapi type, if you change the class without adding an explicit type you get a confusing error:

ArgumentError:
       wrong number of arguments (given 0, expected 1..3)
     # /Users/lrichmond1/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/jsonapi-serializable-0.1.1.beta4/lib/jsonapi/serializable/abstract_resource.rb:78:in `instance_eval'

Defaulting type is one way to fix.

[sumeetattree]

Sorry to piggyback on this issue, but I figured this would be an appropriate place for this question.

We are currently using ams, but have run into some snags with rendering relationships properly. We want to move away from ams but we will have to do it in passes. AMS at the moment is integrated heavily in both, controller rendering and as standalone using AMS::SerializableResource with action cable.

We would like to start by replacing one serializer at a time.

• Good news: jsonapi-rb uses different class names so no conflict with ams
• Bad news: I see that both ams and jsonapi-rb use the same render :jsonapi construct for rendering. Is there a way in which we can make jsonapi-rb use another method? Or are we thinking about this completely the wrong way?