Update README.md

I'm a bit embarrassed to admit that it took me a while to figure out this was a possibility, so I figured maybe this change will save some other poor chaps their time and/or pride.
Update README.md
Create new section 'Disabling root' in README
Merge pull request #301 from dv/patch-1

Update README.md
Update README.md

Setting AMS::Serializer.root = false doesn't automatically disable root for the ArraySerializer.
Merge pull request #305 from dv/patch-2

Clarify disabling root for ArraySerializers
Define serializer as DefaultSerializer if not set
Move reusable code to a module
Make Serializer reuse Serializable
Make include_meta and meta_key private
Add docs to serializable
Move caching to a new module
Get rid of refine
Associations doesn't depend on source serializer anymore
Remove option method just use the reader
Passing options[:hash] is not public API of include!
Extract id_key to a method
serialize_ids doesn't use source serializer and it's object
Move if object to the top
Save result of calling associated_object in a local var