Update README.md doc for .evaluateWithJsonBody()

Hello @dpup, 

Please review the following commits I made in branch 'xiao-update-README'.

271cb5612a27b590c8a7487aace4224db218fa90 (2013-06-27 14:11:17 -0700)
Update README.md doc for .evaluateWithJsonBody()


R=@dpup