License the project under AGPLv3

All significant contributors agreed with this licensing choice, see
https://code.ffdn.org/FFDN/coin/issues/152

README.md

@@ -22,6 +22,11 @@ The project page (issue, wiki, etc) is here:
 
 A mailing list is available, for both users and developers: https://listes.illyse.org/wws/info/si
 
+License
+=======
+
+Coin is released under the GNU Affero GPL 3.0 license.  See `LICENSE` for details.
+
 Extending Coin
 ==============