add breaking change note to CHANGELOG

regarding
https://github.com/emberjs/data/pull/2460
So, apparently this will be a breaking change after all and this part is correct. Talked to @igorT on IRC and we want to provide a gist of how to restore the old behavior. I've created one here (and tested it locally): https://gist.github.com/wecc/3d30c73b5bec4fe3d708