document constructor chaining semantics

[gregbanks]

it's not obvious from the REAME that arguments to a class' constructor are not passed on to a superclass' constructor when atom automatically chains constructor calls. i would assume that arguments would be passed with automatic chaining, otherwise all superclass constructors (and all constructors for classes that may be subclassed in the future) have to behave like default constructors. if this is a requirement, then maybe this should be documented. on the flip side, if arguments are passed, then you end up with longer and longer parameter lists as you work your way through the class hierarchy with each subclass being beholden to the signature of its superclass constructor... also not ideal.

in any case, it seems like the final decision as to how this should work has not been made: https://github.com/carbon-io/atom/blob/master/lib/atom.js#L242