Update cavapapa.ino

Add Licence (GPLv3)

code/cavapapa.ino

@@ -1,3 +1,21 @@
+/*
+Cave_a_Papa
+Copyright (C) 2017  Pierrick C.
+
+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 version 3 of the License.
+
+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/>
+*/
+
 #include <Adafruit_NeoPixel.h>
 #ifdef __AVR__
   #include <avr/power.h>