Update howto with reference to Makefile.

HowToAddaNewMeter.txt

@@ -73,6 +73,9 @@ To add a meter type, make a copy of meter_supercom587.cc to
 meter_mymeter.cc and add your meter to LIST_OF_METERS in meters.h
 and add the driver lookup values (mfct,type,ver) to METER_DETECTION.
 
+Add the line $(BUILD)/meter_mymeter.o \
+to the Makefile.
+
 In meter_mymeter.cc, search and replace supercom587 for mymeter
 possibly adding a new meter type in meters.h if your meter is not a
 water meter and inheriting from that new meter type instead.