add proposal for adding the dest to federation auth headers

Author: jcgruenhage

proposals/3383-fed-auth-destination.md

@@ -0,0 +1,19 @@
+# MSC3383: Include destination in X-Matrix Auth Header
+
+Currently, a federation request can't be validated mid-flight without talking to
+both the originating server and the destination server. The originating server
+needs to be asked for it's key, and the destination server for it's
+`server_name`, because the `Host` header does not necessarily contain the
+`server_name` of the destination server, if the delegation via `.well-known` is
+being used.
+
+## Proposal
+
+## Potential issues
+
+## Alternatives
+
+## Security considerations
+
+## Unstable prefix
+