BLE: Add raw messages

docs/ble-protocol.md

@@ -101,3 +101,11 @@ Repetitions of the same message should be dropped by the BLE server.
 Consecutive messages with no relevant changes may be dropped by the BLE server while keeping some minimal publishing rate, e.g.:
 - only publish MAPEMs every 10 seconds (since content doesn't change, but client may be restarted after first MAPEM from an intersection was received)
 - drop high-frequency CAMs (only publish "full" CAMs which include the low-frequency container)
+
+Maximum message size (assuming realistic value ranges):
+
+field name            | tag size | (max.) data size
+--------------------- | -------- | ---------
+msg_type              |        1 | 1 (VARINT(21))
+payload               |        1 | 2+1394 (LEN(1394) + payload)
+**SUM**               |      2 + | 1397 = 1399 byte