Deprecation warning for synapse admin api being accessible under /_matrix

Author: anoadragon453

CHANGES.md

@@ -1,7 +1,19 @@
 Synapse 1.20.0 (2020-09-22)
 ===========================
 
-No significant changes.
+No significant changes since v1.20.0rc5.
+
+Removal warning
+---------------
+
+Historically, the [Synapse Admin
+API](https://github.com/matrix-org/synapse/tree/master/docs) has been
+accessible under the `/_matrix/client/api/v1/admin`,
+`/_matrix/client/unstable/admin`, `/_matrix/client/r0/admin` and
+`/_synapse/admin` prefixes. In a future release, we will soon be dropping
+support for accessing Synapse's Admin API using the `/_matrix/client/*`
+prefixes. This is to help make locking down external access to the Admin API
+endpoints easier for homeserver admins.
 
 
 Synapse 1.20.0rc5 (2020-09-18)