[MLton] basis html

[Stephen Weeks]

> MLton $(VERSION) implements the Standard ML Basis Library specification as
> of $(BASIS_DATE).  Please see
> http://www.standardml.org/Basis/history.html, for changes to the
> specification that may have occured since $(BASIS_DATE).

I'll add something like this to the user guide too.

But I don't think it's advisable to ask users to spend their time
looking at a spec and working back through the change history to see
what the real spec is.  Just look at how many changes there have been
since MLton 20030716.  We know about them because we're so close and
have to implement them.  But to a user reading the spec to do that
would significantly increase their effort.