Maintain Release Notes

[cburstedde]

Maintain a current and complete Relase Notes file

There is a doc/release_notes.txt file in libsc, which I would encourage everyone to update synchronously to their proposed changes to the code. For p4est, I've just added a mostly empty version to the develop branch.

It will be helpful to our collaborators and users to add to this file with every PR or other addition to the code.

[sloede]

Hi Carsten, I think this is a great addition! And will certainly make our (downstream maintainer) lives a little easier!

One suggestion, if I may: if you make it a markdown file instead of a plain text file, it is a little nicer to view on GitHub and might also encourage people to include, eg, links to relevant PRs. But a plain text file is also OK, of course.

[cburstedde]

We've tried the latest release 2.8.6 (out now!) as is and may look into markdown in the future. Thanks!