layout changes to _layoutFile

[dumingxin]

I saw this in the index.js file:
// for backward-compatibility, allow options to
// set a default layout file for the view or the app
// (NB:- not called layout any more so it doesn't
// conflict with the layout() function)
when use res.render function, the paramter {layout:'layout'} changes to {_layoutFile:'layout'}
it's important message,but I can't see it in the documents,why not add it to the readme.md file

[JacksonTian]

Pull request welcome