Fix #21121: Add archive of security issues.

docs/releases/index.txt

@@ -112,6 +112,16 @@ Pre-1.0 releases
    0.96
    0.95
 
+Security releases
+=================
+
+Whenever a security issue is disclosed via :ref:`Django's security
+policies <internals-security>`, appropriate release notes are now
+added to all affected release series.
+
+Additionally, :ref:`an archive of disclosed security issues
+<security-releases>` is maintained.
+
 Development releases
 ====================