add .ordering to README

this is indeed available in 1.7.3, but undocumented.

web/docs/api-1.7.0.md

@@ -110,6 +110,9 @@ If name is specified then it will be used to generate legend label.
 #### .filterAll()
 Clear all filters associated with this chart.
 
+#### .ordering([orderFunction])
+Get or set an accessor to order ordinal charts
+
 #### .select(selector)
 Execute in scope d3 single selection using the given selector and return d3 selection result. Roughly the same as:
 ```js
@@ -1083,4 +1086,4 @@ var display1 = dc.numberDisplay("#chart-container1");
 Calculate and return the underlying value of the display
 
 #### .formatNumber([formatter])
-Get or set a function to format the value for the display. By default `d3.format(".2s");` is used.
+Get or set a function to format the value for the display. By default `d3.format(".2s");` is used.