License headers on top of each source file (and README)

I have followed those recommendations : https://www.gnu.org/licenses/agpl-3.0.de.html

@librelois @c-geek @Insoleet

added C-repository label

It seems OK to me. That's what Jonas adviced me to do: take the little part of the AGPL license that sums it up and put it as the header of each file.

mentioned in commit 3d5bb988

merged