[isabelle-dev] Isabelle2021-1 release fork

[Makarius]

Now is the critical moment: I will fork tag Isabelle2021-1-RC3 and fork the
isabelle-dev vs. isabelle-release repository.

After current 2b212c8138a5, the isabelle-dev repository is in an "undefined"
state, until there is a follow-up mail announcing the post-release starting point.

If there are changes in the pipeline for Isabelle2021-1: please send them to
me via mail.

If there are changes for post-release development, please await the
announcement for its re-opening.


	Makarius