Docs describing new version and features

[joaomdmoura]

tl;dr

Update new docs and cover new topics: https://github.com/rails-api/active_model_serializers/tree/master/docs

---

I've being personally pushing a new docs forward: https://github.com/rails-api/active_model_serializers/tree/master/docs
We have being updating it accordingly but there is still a long way to go.

We've an ambitious goal with this doc, cover:

• Usual doubts (avoid unnecessary issue creation)
• Migration between versions (help developer to get up to date)
• How to topics (enabling ppl to quickly find solution and how to use features)

The idea is to update this docs to cover all this.

---

related to issues:

• Add docs for include in Json adapter. #1243

related to PRs:

• Add information about default included associations policy per adapter #1229

[bf4]

ref:

• Updating Documentation for 0.10 #1074
• Discussion: gracefully upgrading #1005

[remear]

I think this is done except for the upgrading docs

[bf4]

@remear Yeah, there's just no canonical issue for this. I mean this should be, but there's a bunch of others.

[beauby]

Closing in favor of #1694.