Add v11 migration reminder

mass_mailing_custom_unsubscribe/README.rst

@@ -48,7 +48,9 @@ Known issues / Roadmap
 * This module adds a security hash for mass mailing unsubscription URLs, which
   disables insecure URLs from mass mailing messages sent before its
   installation. This can be a problem, but anyway you'd get that problem in
-  Odoo 11.0, so at least this addon will be forward-compatible with it.
+  Odoo 11.0, where https://github.com/odoo/odoo/pull/12040 was merged, so at
+  least this addon will be forward-compatible with it. So, **this feature must
+  be removed from here when migrating to v11**.
 * This module replaces AJAX submission core implementation from the mailing
   list management form, because it is impossible to extend it. When
   https://github.com/odoo/odoo/pull/14386 gets merged (which upstreams most