add license text at the top of the file

git_slackbot.rkt

@@ -1,6 +1,20 @@
 #! /usr/bin/racket
 #lang racket
 
+ ; Copyright (C) 2016 Wesley Kerfoot
+ ; 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 3 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.
+ ;
+ ; You should have received a copy of the GNU General Public License
+ ; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
 (require json)
 (require net/url)