Currently this is not documented so it is not clear what versioning and deprecation policy DJA follows.

This needs to be documented. I would suggest we follow common standards, using Semver and implement a similar deprecation policy like Ember (which is most likely the most used JS framework with JSON API spec).

This way we have a simple migration guide for our users. We can simply say, upgrade to the latest minor version of a series (e.g. 2.6.0) and solve all deprecation warnings. In version 3.0.0 we simply remove all deprecation warnings and do nothing else (real changes follow then in a version 3.1.0 etc.).

Related to #467 #469