Add license information to README

fixes #260

README.md

@@ -167,3 +167,16 @@ RUN set -ex; \
  git \
  ; \
 ```
+
+# License
+
+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.
+
+For more details about licensing and the exceptions for skins and plugins
+see [roundcube.net/license][license].