CHANGELOG.md: new release notes format

[Lubomir I. Ivanov]

hello,

we've investigated how can we avoid ReleaseNotes.txt conflicts as
nowadays we can have more than one pull requests at Github modifying
the file. we found out that our current RelaseNotes.txt would not work
with a git's `union` attribute that can potentially allows us to
modify the same file from different commits without conflicts

see this change:
https://github.com/Subsurface-divelog/subsurface/pull/883/commits/6b097accfc525a99cec227e3cf72a583cf95d072

what has changed:
- a new file was introduced - /CHANGELOG.md
- make sure to add your release notes ***on top*** of the file
/CHANGELOG.md - i.e. on the first line.

CCing some folks that have pending PR.

thanks
lubomir
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