Add upgrade notes

Author: reivilibre

docs/upgrade.md

@@ -91,6 +91,16 @@ process, for example:
 
 # Upgrading to v1.64.0
 
+## Deprecation of the ability to delegate e-mail verification to identity servers
+
+Synapse v1.66.0 will remove the ability to delegate the tasks of verifying email address ownership, and password reset confirmation, to an identity server.
+
+If you require your homeserver to verify e-mail addresses or to support password resets via e-mail, please configure your homeserver with SMTP access so that it can send e-mails on its own behalf.
+[Consult the configuration documentation for more information.](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#email)
+
+The option that will be removed is `account_threepid_delegates.email`.
+
+
 ## Changes to the event replication streams
 
 Synapse now includes a flag indicating if an event is an outlier when