Update usage

Author: mganss

README.md

@@ -148,6 +148,9 @@ Options:
                                (default is true)
       --nc, --netCore        generate .NET Core specific code that might not
                                work with .NET Framework (default is false)
+      --ar, --useArrayItemAttribute
+                             use ArrayItemAttribute for sequences with single
+                               elements (default is true)
       --ca, --commandArgs    generate a comment with the exact command line
                                arguments that were used to generate the source
                                code (default is true)
@@ -365,4 +368,4 @@ Contrbutions are welcome. Here are some guidelines:
 
 - If it's not a trivial fix, please submit an issue first
 - Try and blend new code with the existing code's style
-- Add unit tests
+- Add unit tests