legal: add COPYRIGHT to mention the contributors
Add a file to mention copyright
from contributors
to the project.
See recommandations from the linux foundation:
This file would explain and validate the copyright holder such as
Copyright The GStreamer Contributors.
Instead of having multiple copyright holders better to follow the VCS to get the contributions.
See gstreamer-project#86 as a start of the discussion.
Freely inspired from Rust COPYRIGHT.
Edited by Stéphane Cerveau