Update README.md for origin function callback parameters

[dstudzinski]

Updated README to better explain which types of parameters can be used for callback when origin is a function.

From what I checked callback's second parameter can be [bool|string] at least (I believe it support more types).

For example it can be used like this:

cors({
  'origin': function (origin, callback) {
    callback(null, origin || '*');
  },
})

To send * as origin when Origin headers is missing and value from Origin header when it's present.

[dougwilson]

Thank you. The second argument is actually any value (except function) that the original origin can be set to.