Warn that /register will soon require a type when called with an access token (#9559)

This notice is giving a heads up to the planned spec compliance fix https://github.com/matrix-org/synapse/pull/9548.

CHANGES.md

@@ -1,3 +1,9 @@
+Removal warning
+---------------
+
+Note that this release deprecates the ability for appservices to call `POST /_matrix/client/r0/register`  without the body parameter `type`. Appservice developers should use a `type` value of `m.login.application_service` as per the spec. In future releases, calling this endpoint with an access token but
+without a valid type will fail.
+
 Synapse 1.29.0 (2021-03-08)
 ===========================
 

changelog.d/9559.removal

@@ -0,0 +1 @@
+Registering an Application Service user without using the `m.login.application_service` login type will be unsupported in an upcoming Synapse release.