updated readme

NEED/README.md

@@ -20,6 +20,7 @@ Issues:
  * 0 = off gas/elec ? 
  * V = valid reading (gas range 0 - 50,000; elec range = 100 - 25,000)
  * L = large (> 50k or 25k depending?)
- * M = missing? But G?
+ * M = missing?
+ * G = ?
 * ideally DECC should set missing to -99 to aid re-coding and avoid unpleasant surprises in naive analysis!