Missing req.body introduction in main page of koa.
Hey ~

I'm newbie to koa, after I walked through koa main page introduction (http://koajs.com/# ), I found after all the detail explanation of ctx.request, there is no introduction about req.body, is this because koa discourage the usage of POST request ?  Or is there any other concern not to expose this in the document ? 

Thanks,
Jerry

