documenting reading numbers with leading zeros

Author: tilo

docs/data_transformations.md

@@ -26,6 +26,13 @@ It removes any values which are `nil` or would be empty strings.
 `convert_values_to_numeric` is enabled by default. 
 SmarterCSV will convert strings containing Integers or Floats to the appropriate class.
 
+Here is an example of using `convert_values_to_numeric` for numbers with leading zeros, e.g. ZIP codes:
+
+```
+  data = SmarterCSV.process('/tmp/zip.csv',  convert_values_to_numeric: { except: [:zip] })
+   => [{:zip=>"00480"}, {:zip=>"51903"}, {:zip=>"12354"}, {:zip=>"02343"}]
+```   
+
 ## Remove Zero Values
 `remove_zero_values` is disabled by default.
 When enabled, it removes key/value pairs which have a numeric value equal to zero.