updated readme re 'valid' codes

NEED/README.md

@@ -16,6 +16,6 @@ Notes (mostly to self):
 * UPRN = unique property reference = linkage mechanism (uses AddressBase)
 
 Issues:
-* Gcons<year>valid variable has undefined labels: G, L, M = ?  0 = off gas & V = valid reading (range 0 - 50,000) so presumably L = large (> 50,000?) and M = missing? But G?
+* <fuel>cons<year>valid variable has undefined labels: G, L, M = ?  0 = off gas/elec & V = valid reading (gas range 0 - 50,000; elec range = 100 - 25,000) so presumably L = large (> 50k or 25k depending?) and M = missing? But G?
 * ideally DECC should set missing to -99 to aid re-coding and avoid unpleasant surprises in naive analysis!