Update README.md

This is minor, but the docs suggest that the first three arguments of `sourceNode` are required when in reality [they all seem to be optional](https://github.com/mozilla/source-map/blob/master/lib/source-map/source-node.js#L27-L35), by which I mean that no error is thrown when you simply have `var x = new SourceNode();` before working with the node.