Update the DEPRECATIONS file

Author: ibmmqmet

DEPRECATIONS.md

@@ -2,4 +2,8 @@
 No immediate plans for anything specific.
 
 However it is likely that at some indeterminate future time, some of the lesser-used collectors
-will be removed: aws, influx, opentsdb being most likely.
+will be removed: aws, influx, opentsdb being most likely.
+
+I also expect to change the Prometheus collector's default value of the `overrideCType` attribute, to
+more accurately reflect the different between Counters and Gauges. That may mean reworking some of your
+dashboards.