chore(debian): Add GPL-3.0 copyright file for debian packaging

Author: mmstick

debian/copyright

@@ -0,0 +1,24 @@
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: pop-launcher
+Upstream-Contact: System76 <info@system76.com>
+Source: https://github.com/pop-os/launcher
+
+Files: *
+Copyright: 2021 System76 <info@system76.com>
+License: GPL-3.0
+ This software 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 3 of the License, or (at your option) any later version.
+ .
+ This software 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.
+ .
+ You should have received a copy of the GNU General Public
+ License along with this software; if not, write to the Free Software
+ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
+ .
+ On Debian systems, the complete text of the GNU General
+ Public License can be found in `/usr/share/common-licenses/LGPL-3'