Add mention that .all()'s fulfilment values have ordered maintained

I was wondering if .all() maintained the order of the fulfilment values for each promise 
despite different promises resolving at different points in time.

I couldn't find this mentioned in the README so I wrote a personal test to find out 
and I've concluded that it's the case so I've mentioned it in the README.

```javascript

// Testing if .all() maintains the order of the returned values

var q = require('q');

function soon (id, time) {
  var deffered = q.defer();
  setTimeout(function () {
    console.log(id + ' is done');
    deffered.resolve(id);
  }, time);
  return deffered.promise;
}

q.all([
  soon("a", 5000),
  soon("b", 100),
  soon("c", 2000)
])
.then(function (results) {
  console.log(results); // should be a, b, c and not b, c, a
});
```