docs/manual: add info about AGPLv3 license

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/legal-notice.txt

@@ -74,6 +74,9 @@ file to inform you of relevant material that could not be saved.
 Here is a list of the licenses that are most widely used by packages in
 Buildroot, with the name used in the manifest files:
 
+* `AGPLv3`:
+  http://www.gnu.org/licenses/agpl-3.0.en.html[
+  GNU Affero General Public License, version 3];
 * `GPLv2`:
   http://www.gnu.org/licenses/old-licenses/gpl-2.0.html[
   GNU General Public License, version 2];