Add statement about source code availability for Prolog Cafe

EPL ยง3 states:

  A Contributor [i.e.: Gerrit Code Review] may choose to distribute
  the Program in object code form under its own license agreement,
  provided that:
  [...]
  b) its license agreement:
  [...]
  iv) states that source code for the Program [i.e.: Prolog Cafe] is
  available from such Contributor, and informs licensees how to obtain
  it in a reasonable manner on or through a medium customarily used
  for software exchange.

Change-Id: Ie002e0baf560fe219838e3bd49d5a103a75f44db
diff --git a/Documentation/licenses.txt b/Documentation/licenses.txt
index 549620b..69a12c3 100644
--- a/Documentation/licenses.txt
+++ b/Documentation/licenses.txt
@@ -408,7 +408,8 @@
 
 Originally developed by Mutsunori BANBARA and Naoyuki TAMURA at the
 Kobe University, JAPAN. Gerrit Code Review uses a fork derived from
-the 1.2.5 release.
+the 1.2.5 release, and offers the corresponding source code at
+link:https://gerrit.googlesource.com/prolog-cafe[].
 
 Prolog Cafe is dual licensed and available under either the
 link:http://opensource.org/licenses/eclipse-1.0.php[Eclipse Public License],