Add documentation URL to repo description

[GenevieveBuckley]

This is different to #67 (but it is related).

@jni you can add a URL to the right hand sidebar description on your repo page. Can you add the documentation url there? https://jni.github.io/affinder/

You have to look for the little gear icon in the right hand sidebar, next to the "About" repo info. You click it to edit the repo description and url.

Then there's a spot you can add the URL

[jni]

Done, thanks! 🙏