Relicense documentation under CC-BY-4.0

All contributors whose contributions were still being used
at the moment of relicensing have agreed to it.

See #2146 .

COPYING

@@ -5,6 +5,13 @@ copy of the license file as AGPL-3.
 
 ---
 
+Files inside docs directory are copyright © 2021 Pleroma Authors
+<https://pleroma.social/>, and are distributed under the Creative Commons
+Attribution 4.0 International license, you should have received
+a copy of the license file as CC-BY-4.0.
+
+---
+
 The following files are copyright © 2019 shitposter.club, and are distributed
 under the Creative Commons Attribution-ShareAlike 4.0 International license,
 you should have received a copy of the license file as CC-BY-SA-4.0.