[Dune] first release candidate for 2.6 release

[Andreas Dedner]

Hi everyone.

We just wanted to let you know that just in time for Christmas the first
release candidate of the 2.6 release is available for testing.
Tar balls are available
https://www.dune-project.org/releases/2.6.0rc1/
or use the v2.6.0rc1 tag for helping us, by testing the release with
your own modules, system, workflow... Let us know if you encounter any
problems. Also please skim through the change log (same link as above)
to see if something is unclear or missing.

Wish you all a merry Christmas and a happy new year
Andreas and Martin