document the channel name -> channel number hash algorithm

mesh.proto

@@ -430,6 +430,21 @@ message ChannelSettings {
   A channel number between 1 and 13 (or whatever the max is in the current
   region). If ZERO then the rule is "use the old channel name hash based
   algoritm to derive the channel number")
+
+  If using the hash algorithm the channel number will be: hash(channel_name) %
+  NUM_CHANNELS (Where num channels depends on the regulatory region).
+  NUM_CHANNELS_US is 13, for other values see MeshRadio.h in the device code.
+
+  // hash a string into an integer - djb2 by Dan Bernstein. -
+  // http://www.cse.yorku.ca/~oz/hash.html
+  unsigned long hash(char *str) {
+    unsigned long hash = 5381; int c;
+
+    while ((c = *str++) != 0)
+      hash = ((hash << 5) + hash) + (unsigned char) c;
+
+    return hash;
+  }
   */
   uint32 channel_num = 9;