README: Document GetSalt packet.

README.md

 
 # Protocol
 
-Packets are up to 32 bytes long. Most bytes are encrypted via XXTEA, except for
-one packet sent to the base station from a device to request a random number. The
-base station then replies with an encrypted packet containing a random salt
-value, and the device uses this salt for all further communication until the next
-reset, increasing the salt value by one for each packet sent. This scheme removes
-the need for the device to generate random numbers, as microcontrollers commonly
-are not capable of generation of high-quality random numbers.
-
-The packet sent from the device to the base station to request a salt consists of
-a single byte with the sender ID and is recognized by the base station by its
-length.
+Packets are up to 32 bytes long. All bytes are encrypted via XXTEA. First, the
+device requests a salt from the base station requests (using salt 0 for the
+packet). The base station then replies with an encrypted packet containing a
+random salt value, and the device uses this salt for all further communication
+until the next reset, increasing the salt value by one for each packet sent.
+This scheme removes the need for the device to generate random numbers, as
+microcontrollers commonly are not capable of generation of high-quality random
+numbers.
 
 ### Packet Format
 
    The most significant bit of the salt value is 0 for packets from the device
    to the base station and 1 for packets from the base station to the device.
 
+   For GetSalt packets, the remainder of the salt is 0.
+
 All fields from here on are encrypted with XXTEA, using the first 8 bytes of the
 packet as the salt.
 
    The 5 least significant bits of the field encode the packet type, with the
    following possible values:
 
-   0. Salt: The packet is sent by the base station to the device whenever a
+   0. GetSalt: The packet is sent by the device when no salt is available. The
+      base station replies with a Salt packet.
+   1. Salt: The packet is sent by the base station to the device whenever a
       1-byte packet containing the device ID is received.
-   1. Report: The packet contains values measured by _this_ node. The element
+   2. Report: The packet contains values measured by _this_ node. The element
       count signals the number of values (of different types) reported by the
       device.
-   2. GetValues: Sent by a device to request values generated by _other_ sensor
+   3. GetValues: Sent by a device to request values generated by _other_ sensor
       nodes from the base station, or to request the current time. The element
       count signals the number of different values requested.
-   3. Values: Reply to a `GetValues` packet containing the requested information.
+   4. Values: Reply to a `GetValues` packet containing the requested information.
 
    The 3 most significant bits of the field encode the element count for packets
    which can hold multiple elements (as described below).