add compatibility clause for Erlang Public License

This is required to make sure that even if somebody thinks the erlang
OTP libraries and my code form a derivative work, there will be no
licensing conflict.
diff --git a/src/m3ua_core.erl b/src/m3ua_core.erl
index 0701954..3167c81 100644
--- a/src/m3ua_core.erl
+++ b/src/m3ua_core.erl
@@ -16,6 +16,20 @@
 %
 % You should have received a copy of the GNU Affero General Public License
 % along with this program.  If not, see <http://www.gnu.org/licenses/>.
+%
+% Additional Permission under GNU AGPL version 3 section 7:
+%
+% If you modify this Program, or any covered work, by linking or
+% combining it with runtime libraries of Erlang/OTP as released by
+% Ericsson on http://www.erlang.org (or a modified version of these
+% libraries), containing parts covered by the terms of the Erlang Public
+% License (http://www.erlang.org/EPLICENSE), the licensors of this
+% Program grant you additional permission to convey the resulting work
+% without the need to license the runtime libraries of Erlang/OTP under
+% the GNU Affero General Public License. Corresponding Source for a
+% non-source form of such a combination shall include the source code
+% for the parts of the runtime libraries of Erlang/OTP used as well as
+% that of the covered work.
 
 -module(m3ua_core).
 -author('Harald Welte <laforge@gnumonks.org>').