Add GPL license comment section.

sketch/sketch.ino

@@ -1,6 +1,24 @@
 #include <Chrono.h>
 #include <LightChrono.h>
 
+/*
+Convert wireless playstation guitar to USB guitar with Teensy.
+Copyright (C) 2020  Adam Bissen
+
+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/>.
+*/
+
 /*
 Need 3.3V supply to touch controls. Voltage feedback for each section
 #Whammy Bar. 3.3V supply to pot