add support flag to /versions

proposals/3881-remote-push-notification-toggling.md

@@ -58,11 +58,19 @@ MSC.
 
 ## Migration
 
+Home servers should indicate their support of this MSC by adding `org.matrix.msc3881` and `org.matrix.msc3881.stable` in
+the response of `/_matrix/client/versions`.
+
 Clients that connect to a home server that doesn't yet support this proposal should interpret a missing `enabled` value
 as `true`.
 
 Home servers should migrate pushers that were registered before this proposal so that `enabled` is `true`
 
+
+
+# Adds support for remotely enabling/disabling pushers, as per MSC3881
+                    "org.matrix.msc3881": self.config.experimental.msc3881_enabled,
+
 ## Potential issues
 
 Adding an enabled state to pushers increases the complexity of the push notification API. In addition to a pusher