matrix-doc/event-schemas/examples/v1/m.receipt

{
    "type": "m.receipt",
    "room_id": "!KpjVgQyZpzBwvMBsnT:matrix.org",
    "content": {
        "$1435641916114394fHBLK:matrix.org": {
            "read": {
                "@rikj:jki.re": {
                    "ts": 1436451550453
                }
            }
        }
    }
}