Add proposal for invite error code

proposals/1866-invite-unsupported-version-error-code.md

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+# MSC 1866 - Unsupported Room Version Error Code for Invites
+
+It is currently unspecified what error code should be relayed to clients when
+they attempt to invite a user on a remote server that does not support the room
+version.
+
+The proposal is to reuse the `M_UNSUPPORTED_ROOM_VERSION` error code that is
+currently returned by the create room API.
+
+Strictly, the error returned by the create room API would mean the local server
+didn't support the room version, while for the invite API it would mean the
+remote server didn't. However, there is sufficient overlap that it makes sense
+to reuse the same error code and rely on the context to differentiate the two
+cases.