Write Filtering of /publicRooms by room type

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

proposals/3827-space-explore.md

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+# MSC3827: Filtering of `/publicRooms` by room type
+
+[MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772) defines Spaces as
+rooms with `type` of `m.space`. One of the most requested features for Spaces is
+to have [a way to to discover
+them](https://github.com/vector-im/element-web/issues/17264). To solve this
+issue, this MSC proposes a mechanism for filtering the response of
+`/publicRooms` by room type.
+
+## Proposal
+
+This MSC proposes adding a `room_type` filter to the `/publicRooms` endpoint.
+The value of `room_type` is an array of room types - possible values of the
+`type` field in the `m.room.create` state event - which should be returned.
+
+An example request could look like this:
+
+```json
+POST _matrix/client/v3/publicRooms
+{
+  "filter": {
+    "room_type": ["m.space"]
+  },
+  "limit": 10
+}
+```
+
+the response then also includes a `room_type` field, so that clients can show
+the room type metadata.
+
+```json
+{
+    "chunk": [{
+        "avatar_url": "mxc://bleecker.street/CHEDDARandBRIE",
+        "guest_can_join": false,
+        "join_rule": "public",
+        "name": "CHEESE",
+        "num_joined_members": 37,
+        "room_id": "!ol19s:bleecker.street",
+        "topic": "Tasty tasty cheese",
+        "world_readable": true,
+        "room_type": "m.space"
+    }],
+    "next_batch": "p190q",
+    "prev_batch": "p1902",
+    "total_room_count_estimate": 115
+}
+
+```
+
+## Alternatives
+
+### Using a special endpoint
+
+A special endpoint for spaces and other room types could be used. Then each new
+room type would require a special MSC and there would be no way to filter by
+custom room types. Filtering by multiple types would be impossible.
+
+### Replacing the room directory with a server-wide space
+
+[A matrix-spec issue](https://github.com/matrix-org/matrix-spec/issues/830)
+suggests replacing the room directory with a server-wide space; while this
+solution certainly has benefits, it would require a lot of changes both on the
+client and the server side.
+
+## Unstable prefix
+
+While this MSC is not considered stable, implementations should use
+`org.matrix.msc3827` as a namespace.
+
+|Release    |Development                   |
+|-----------|------------------------------|
+|`room_type`|`org.matrix.msc3827.room_type`|