Add copyright

debian/copyright

+Format: http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: Icinga L10n
+Source: https://github.com/Icinga/L10n
+Homepage: https://www.icinga.com
+
+Files: *
+Copyright: 2020 Icinga GmbH <info@icinga.com>
+License: GPL-2
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 2 of the License, or
+ (at your option) any later version.
+ .
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ GNU General Public License for more details.
+ .
+ On Debian systems, the full text of the GNU General Public
+ License version 2 can be found in the file
+ `/usr/share/common-licenses/GPL-2'.