Updated README to include tm format

Optimally this would be calculated before being published to Navdata,
but for backwards compatibility lets document the format for now, and
discuss an additional unpacked time

README.md

@@ -86,7 +86,7 @@ Information received from the drone will be published to the `ardrone/navdata` t
 * `altd`: Estimated altitude (mm)
 * `vx`, `vy`, `vz`: Linear velocity (mm/s) [TBA: Convention]
 * `ax`, `ay`, `az`: Linear acceleration (g) [TBA: Convention]
-* `tm`: Timestamp of the data returned by the Drone
+* `tm`: Timestamp of the data returned by the Drone returned as a packed uint32 (sec:11; usec:21)
 
 ### IMU data