Demonstrate using ExpressReceiver's router property to add custom HTTP routes

Description

Many developers have asked how they can add new HTTP routes to their Bolt app. These routes are usually unrelated to Slack, but used to handle webhooks from other APIs, serve an API, or to render webpages.

Since v2.1.0, this is a supported feature of ExpressReceiver via the added router property. That property exposes an Express Router on which more routes can be added.

This information is not well documented today, and it should be. Here is a starting code example that could be used:

const { App, ExpressReceiver } = require('@slack/bolt');

// Create a Bolt Receiver
const receiver = new ExpressReceiver({ signingSecret: '' });

// Create the Bolt App, using the receiver
const app = new App({ token: '', receiver });

// Slack interactions are methods on app
app.event('message', async ({ event, client }) => {
  // Do some slack-specific stuff here
  await client.chat.postMessage(...);
});

// Other web requests are methods on receiver.router
receiver.router.post('/secret-page', (req, res) => {
  // You're working with an express req and res now.
  res.send('yay!');
});

(async () => {
  await app.start(8080);
  console.log('app is running');
}());

Ref: https://github.com/slackapi/bolt-js/issues/283#issuecomment-600035307

Requirements (place an x in each of the [ ])

• I’ve read and understood the Contributing guidelines and have done my best effort to follow them.
• I’ve read and agree to the Code of Conduct.
• I’ve searched for any related issues and avoided creating a duplicate issue.

[stevengill]

The doc is now live at https://slack.dev/bolt-js/concepts#custom-routes

[szgaljic]

I totally missed that step! It works now, thanks!