Add a file CHANGELOG.md in the root Subsurface directory
and track this file with the `-text merge=union` attribute.
This prevents merge conflicts if release notes are added
on top of the file by different commits.
Also, make ReleaseNotes/ReleaseNotes.txt point to that file.
Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
