Update README.md

README.md

 # locapack
 
+
+
+## Packet structure details
+
+### Packet general structure
+
+|    2B    |         2B        |    1B (opt)   |     variable     |            variable           |
+|:--------:|:-----------------:|:-------------:|:----------------:|:-----------------------------:|
+| `header` | `sequence_number` | `movement_id` | `source_address` | `localisation_packet_payload` |
+
+The `sequence_number` field is mandatory. 
+
+The `movement_id` field is optional and its presence is announced in the header.
+
+The `source_address` field length is annouced in the header. A length of 0 means that the field is not present in the packet.
+
+The `localisation_packet_payload` length depends on the `localisation_packet_type` annouced in the header.
+
+
+### Header structure
+
+| 4b                 | 1b                          | 4b                    | 4b                         | 3b       |
+|--------------------|-----------------------------|-----------------------|----------------------------|----------|
+| `protocol_version` | `movement_id_presence_flag` | `source_address_size` | `localisation_packet_type` | reserved |
+
+The `protocol_version` field indicates the version of the localisation packet used. This documentation describes the version #1 of the localisation packet protocol.
+
+The `movement_id_presence_flag` must be set to 1 if the `movement_id` field is present in the packet.
+
+The `source_address_size` indicates the number of Bytes of the adresse of the sender. A length of 0 means that no source address is present in the packet.
+
+The `localisation_packet_type` indicates the structure of the payload. These types of packet have been described:
+
+  * `type 0` : Universal GNSS localisation
+  * `type 1` : Locally referenced localisation
+
+### Payload structure by packet type
+
+#### Type 0 : Universal GNSS localisation
+
+#### Type 1 : Locally referenced localisation