Spec reference relationships (#1206)

* Spec reference relationships

MSC: https://github.com/matrix-org/matrix-spec-proposals/pull/3267

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Edits per code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

content/client-server-api/modules/reference_relations.md

@@ -0,0 +1,55 @@
+---
+type: module
+---
+
+### Reference relations
+
+{{% added-in v="1.4" %}}
+
+Generically referencing another event can be done with a `rel_type` of `m.reference`
+as a form of [relationship](#forming-relationships-between-events). There is no
+implied meaning behind the reference, and is usually context-dependent. One
+example is the [key verification framework](#key-verification-framework) which uses
+reference relations to associate distinct events with a specific verification attempt.
+
+{{% boxes/note %}}
+Clients which wish to use threads or replies are expected to use other relationship
+types than references. References are typically used to associate data rather than
+messages.
+{{% /boxes/note %}}
+
+#### Server behaviour
+
+##### Server-side aggregation of `m.reference`
+
+The aggregation format of `m.reference` relations consists of a single `chunk` property,
+which lists all the events which `m.reference` the event (the parent). Currently,
+only a single `event_id` field is present on the events in the `chunk`.
+
+An example `m.reference` would be:
+
+```json
+{
+  "content": {
+    "m.relates_to": {
+      "rel_type": "m.reference",
+      "event_id": "$another_event"
+    }
+    // other content fields as required
+  }
+  // other fields as required by events
+}
+```
+
+The [bundle](#aggregations) under `m.relations` would appear similar to the following:
+
+```json
+{
+  "m.reference": {
+    "chunk": [
+      { "event_id": "$one" },
+      { "event_id": "$two" }
+    ]
+  }
+}
+```