release-notes: use CHANGELOG.md

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>

ReleaseNotes/ReleaseNotes.txt

@@ -1,31 +1,10 @@
+Please add release notes on top of the file /CHANGELOG.md
+
 // _Subsurface_ 4.7.5 Release Notes
 
 _Subsurface_ 4.7.5 - Month dd, 2017
 ------------------------------------------
 
-Some of the changes since _Subsurface_ 4.7.4
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-- Limit min. GFlow to 10 and min. GFhigh to 40 in preferences for profile
-  and planner
-- Fix issues related to debug logging on Windows
-- Add "Bluetooth mode" in the BT selection dialog: Auto, LE, Classical
-- Correct display of cylinder pressures for merged dives
-- Allow user defined cylinders as default in preferences (#821)
-- mobile: fix black/white switch in splash screen (#531)
-- UI: tag editing. Comma entry shows all tags (again) (#605)
-- mobile: enable auto completion for dive site entry (#546)
-- Printing: the bundled templates are now read-only and are always overwritten
-  by the application. The first time the user runs this update, backup files
-  of the previous templates would be created (#847)
-- Fix issues with filters not updating after changes to the dive list
-  (#551, #675)
-- map-widget: allow updating coordinates on the map when the user
-  is editing a dive site by pressing Enter or clicking a "flag" button
-- map-widget: prevent glitches when the user is interacting with the map
-  while animations are in progress
-
-
 Some of the changes since _Subsurface_ 4.7.2
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~