Add copyright to debian folder (#12)

Reasoning: The copyrights will be included the debian packages and can be scanned on a deployed VM by Delphix's automation.

Author: sravyamks

debian/copyright

@@ -0,0 +1,23 @@
+Format: http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: drgn
+Source: https://github.com/osandov/drgn
+
+Files: *
+Copyright: Copyright (c) Facebook, Inc. and its affiliates.
+License: GPL v3.0
+
+License: GPL-3
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License version 3, as
+ published by the Free Software Foundation.
+ .
+ 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.
+ .
+ You should have received a copy of the GNU General Public License
+ along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ .
+ The complete text of the GPL version 3 can be seen in
+ /usr/share/common-licenses/GPL-3.