Add license file

Add a file with the complete GPLv3 license text which is required by the GPLv3 license.

changed milestone to %APR21

I see you did the same commit. I will cancel MR-366.

One thing licence is used for the noun; license is used for the verb. American English doesn't distinguish though.

Dear @f.uhlig, @v.friese, @p.-a.loizeau,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

added CodeOwners label

merged