5.7.0 release announcement

[Simon Frei]

I wrote a release announcement for the 5.7.0 release that is already out
there for a while. Can you please have a look at it and do
corrections/extensions of it if needed - would be greatly appreciated.
The formatted version is on our staging site
https://staging.digikam.org/news/2017-09-11-5.7.0_release_announcement/,
changes can be made on our website repo in the dev branch
(https://phabricator.kde.org/source/websites-digikam-org/repository/master/).

Cheers,
Simon