add some exposition to mpu example in README.md

This is re @mikeal's comment here:
https://github.com/mikeal/request/issues/345#issuecomment-13998419

This updates the mpu example in the README with some more comments and a callback to make the usage unmistakably clear for beginners and especially forgetful developers (e.g. @mikermcneil)
made the language clearer

don't want to give anyone any weird ideas
Merge pull request #810 from mikermcneil/patch-1

add some exposition to mpu example in README.md