add license disclaimer at top

listen

@@ -5,6 +5,19 @@
 # listen: a simple configurable build system
 # created by Bryson Steck, @brysonsteck on GitHub
 # free and open source under the GPL Version 3
+#
+# 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 <https://www.gnu.org/licenses/>.
 # 
 # Usage
 # listen [flags] [file(s) to watch] [command(s) to run on file changes]