Call room room type MSC

Signed-off-by: Šimon Brandner <simon.bra.ag@gmail.com>

proposals/0000-call-room-type.md

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+# MSC0000 Call room room type
+
+[MSC3401](https://github.com/matrix-org/matrix-doc/pull/3401) defines how native
+Matrix group calls can work. It allows for immersive voice/video/call rooms.
+[MSC1840](https://github.com/matrix-org/matrix-doc/pull/1840) proposes a way of
+giving rooms types.
+
+We should use `m.room.type` to inform clients about the fact a room is a
+call room, so that we consistently specify the room type.
+
+## Proposal
+
+This MSC proposes that when a room admin wishes a room to be used as a call
+room, they should set an `m.room.type` state event where the `type` is set to
+`m.call`.
+
+### Example
+
+```json
+{
+    "content": {
+        "type": "m.call"
+    },
+    "event_id": "$143273582443PhrSn:example.com",
+    "origin_server_ts": 1432735824653,
+    "room_id": "!jEsUZKDJdhlrceRyVU:example.com",
+    "sender": "@user:example.com",
+    "state_key": "",
+    "type": "m.room.type",
+    "unsigned": {
+        "age": 1234
+    }
+}
+```
+
+## Potential issues
+
+The `m.intent` field in the `m.call` state event could get out of sync with the
+room type.
+
+## Alternatives
+
+Clients could look for the `m.call` state event and its `m.intent` field though
+this feels weird as the room type feels like the place where we should put this.
+
+## Unstable prefix
+
+|Stable  |Unstable                 |
+|--------|-------------------------|
+|`m.call`|`org.matrix.msc0000.call`|