Add proposal for room version 8.

proposals/3289-rooms-v8.md

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+# MSC3289: Room Version 8
+
+A new room version, `8`, is proposed using [room version 7](https://spec.matrix.org/unstable/rooms/v7/)
+as a base and incorporating the following MSCs:
+
+* [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083) - Restricting room
+  membership based on membership in other rooms
+
+Though other MSCs are capable of being included in this version, they do not have
+sufficient implementation to be considered for this room version. A future room
+version may include them.
+
+Room version `8` upon being added to the specification shall be considered stable.
+No other room versions are affected by this MSC. Before room version `8` can enter
+the specification, MSC3083 needs complete its final comment period.