Add release-notes-2-7.md

[jsmeix]

Initial release-notes-2-7.md
This is currently work in progress for the upcoming ReaR 2.7 release
see rear/rear#2751

[jsmeix]

Perhaps the current new
"New features, bigger enhancements, and possibly backward incompatible changes:"
section is too big and/or too detailed.
I will have a look if I could somehow summarize things therein.

[jsmeix]

@gdha @pcahyna @rear/contributors
I would like to merge this pull request
tomorrow afternoon unless there are objections.

[jsmeix]

Now there is
https://relax-and-recover.org/documentation/release-notes-2-7
but it is not yet public accessible via
https://relax-and-recover.org/documentation/
because there is not yet a link to it in
https://github.com/rear/rear.github.com/blob/master/documentation/index.md
which will be added when ReaR 2.7 is actually released

[jsmeix]

I noticed that documentation/index.md
contains several 'http' URLs in links (i.e. no 'https')
cf. #15