Doc tweak

Author: Alan Christie

README.rst

@@ -21,15 +21,25 @@ there::
 
     pip install im-data-manager-job-utilities
 
-Once installed you can use the utilitites using the supplied classes::
+Once installed you can use the utilities using the supplied classes::
 
     >>> from dm_job_utilities.dm_log import DmLog
+    >>> from decimal import Decimal
     >>> DmLog.emit_event('Hello World!')
     2022-02-03T16:39:27+00:00 # INFO -EVENT- Hello World!
-    >>> DmLog.emit_cost(5.7)
+    >>> DmLog.emit_cost(Decimal('5.7'))
     2022-02-03T16:40:16+00:00 # INFO -COST- 5.7 1
 
 
+Costs are *total* by default but can be issued as an incremental value::
+
+    >>> DmLog.emit_cost(Decimal('0.5'), incremental=True)
+    2022-02-03T16:40:16+00:00 # INFO -COST- +0.5 2
+
+
+The final value on the COST line is a unique sequence number. This value
+is required and is used by the Data Manager to avoid duplicating costs.
+
 .. _PyPI: https://pypi.org/project/im-data-manager-job-utilities
 
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