add license header template

Author: alessio-perugini

Diff for: license_header.tpl

@@ -0,0 +1,14 @@
+This file is part of arduino-cli.
+
+Copyright{{ if .Year }} {{.Year}}{{ end }} {{.Holder}}
+
+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 [email protected].