Release Notes are now to be found here

https://totara.community/mod/forum/view.php?id=1834

In addition it is worth looking at the CHANGELOG.txt file in the codebase, and for the 2.6 release we also provided a more detailed document on the new and changed features from 2.5

https://totara.community/mod/forum/discuss.php?d=9360#p41996