The release notes are on a file on the root of Arducopter, but only for release versions.
Dev versions like the future 3.7 have no plan yet of features and when the fork for 3.7 occurs a specific release notes will be created. The amount of new features on master is so large, with some complex features added that giving a rough estimate of what will be available is not realistic at the moment.
Timeframe will also depend of what will be targeted to go on 3.7 @rmackay9 I think I covered the gist.