On Intercom, go to the App Store icon on the left sidebar, search for ReadMe, and click Install.
Once you go through the Intercom OAuth flow, you'll need your ReadMe API key. It's in General Settings > API Key on ReadMe.
Once the app is installed, you can add it to your Messenger by clicking the Messenger icon on the sidebar. Go to Add Messenger home apps, click + Add an app, and choose ReadMe.