Document the chainability.
The `app.use()` function [appears to return](https://github.com/koajs/koa/blob/422e539e8989e65ba43ecc39ddbaa3c4f755d465/lib/application.js#L130) `this`, so it's chainable like so: `app.use(a).use(b).use(c).listen(8888)`

It's not document here: https://github.com/koajs/koa/blob/422e539e8989e65ba43ecc39ddbaa3c4f755d465/docs/api/index.md#appusefunction

I just tested it, it seems to work fine, its return value should be documented.