Add license info to README (mastodon#6583)

* Add license info to README

* Reference AUTHORS file

README.md

@@ -79,6 +79,16 @@ You can open issues for bugs you've found or features you think are missing. You
 
 **IRC channel**: #mastodon on irc.freenode.net
 
+## License
+
+Copyright (C) 2016-2018 Eugen Rochko & other Mastodon contributors (see AUTHORS.md)
+
+This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero 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 Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License along with this program. If not, see <https://www.gnu.org/licenses/>.
+
 ---
 
 ## Extra credits