Welcome to the community @benny!
That should be no problem. You can create an own node that uses that library. Information about how to create a node in n8n can be found here:
If you follow the guide in CONTRIBUTING.md you should be able to easily setup n8n locally. Once that is done you can simply copy an existing node (like NoOp.node.ts) or start from scratch. All that your node would have to do is to use the incoming HTML (which would be supplied by the HTTP Request Node), give that to the readability module and then return the output.
If you do not want to create extra an own node you could also use the HTML-Extract Node. With it, you could extract the information you care about via a CSS-selector and use that one instead.