[IMP] add license file for the mail addins repository

Author: std-odoo

COPYRIGHT

@@ -0,0 +1,15 @@
+
+Most of the files are
+
+  Copyright (c) 2020-2023 Odoo S.A.
+
+Many files also contain contributions from third
+parties. In this case the original copyright of
+the contributions can be traced through the
+history of the source version control system.
+
+When that is not the case, the files contain a prominent
+notice stating the original copyright and applicable
+license, or come with their own dedicated COPYRIGHT
+and/or LICENSE file.
+