best practices example in readme

[nickdesaulniers]

HI there, I've been monitoring memory usage with

top -pid `pgrep -n node`

I've found that if the callback function passed as the second argument closes over another variable it's better to use the pattern:

var obj1;
weak(obj2, function x () {
  use(obj1);
  x = null;
});

Than it is to do:

var obj1;
function x () {
  use(obj1);
};
weak(obj2, x);

For my app, the first stabilizes around 722 MB of memory, while the second stabilizes at 1.8 GB. Not sure why/if this is useful.