Release [2020.04] - Hard feature freeze now effective

Dear RIOTers,

We've created the 2020.04 release branch, so we are now officially in feature freeze. This means from now only bugfixes will be backported to the `2020.04-branch`. If anyone has any bugfixes or documentation improvements that they feel should be part of this release please let me know.

New features can still be merged into master during this period.

The tracking issue of Release candidate 1 [1] is opened. If you want to help with testing, please put your name against the corresponding item in the linked spreadsheet.

Thanks all for the awesome work!