Ah, the file does not exist in the master yet. An example can be found here: https://github.com/forkcms/forkcms/blob/filesystem/UPGRADE_3.5.md (note filename should contain 3.6 not 3.5). I like how Symfony documents these changes aswel: https://github.com/symfony/symfony/blob/master/UPGRADE-2.2.md

You are changing the behavior of a method (very drastically), this should be documented (anyone could be using it in custom modules).