borg 2.0 release announcement

[ThomasWaldmann]

prepare a release announcement with a relatively short changes list:

https://www.borgbackup.org/releases/

Info sources:

• https://github.com/borgbackup/borg/blob/master/docs/changes.rst
• borg2: work in progress... #6602
• https://github.com/borgbackup/borg/issues?q=label%3Abreaking

[ThomasWaldmann]

https://www.borgbackup.org/releases/borg-2.0.html