add license to global.go

Author: Roberto Sora

global/global.go

@@ -1,3 +1,20 @@
+/*
+ * This file is part of arduino-cli.
+ *
+ * Copyright 2018 ARDUINO SA (http://www.arduino.cc/)
+ *
+ * This software is released under the GNU General Public License version 3,
+ * which covers the main part of arduino-cli.
+ * The terms of this license can be found at:
+ * https://www.gnu.org/licenses/gpl-3.0.en.html
+ *
+ * You can be released from the requirements of the above licenses by purchasing
+ * a commercial license. Buying such a license is mandatory if you want to modify or
+ * otherwise use the software for commercial activities involving the Arduino
+ * software without disclosing the source code of your own applications. To purchase
+ * a commercial license, send an email to license@arduino.cc.
+ */
+
 package global
 
 import (