update readme to reflect `next()` promise return
update readme to reflect `next()` promise return

 #234 points out that the example does not yield to the next middleware
 when adding additional and returning a promise. The `next` argument
 must be resolved and added to the promise chain in order for middleware
 execution to continue.

 ex:

```
 router.use(function(ctx, next) {
   return new Promise(function(resolve, reject) {
     setTimeout(function() {
       // must resolve( { the next middleware } )
       // to continue the promise chain.
       resolve(next());
     }, 100);
   });
 }, function(ctx, next) {
   // will now be called
 });
```

alexmingoia/koa-router,alexmingoia/koa-router,alexmingoia/koa-router