Update readme with APIs for adding/parsing custom mime types in Rails 5

[unicornzero]

Option 1:
Asapplication/vnd.api+json requests are parsed like standard application/json, we do not need to register a separate mime type. Documentation has therefore been added to show how to add a synonym for json parsing without creating a separate mime type.

Option 2:
Per the Rails changelog the correct API for adding parameter parsers is ActionDispatch::Request.parameter_parsers=. Not using this API can lead to conflicts when an application needs to add other parameter parsers or mime types.

Both approaches work.

[djones]

🍍 LGTM

[djones]

Congrats on your first contribution @unicornzero!