A question of etiquette?

I am updating (forking) a WP plugin over on Github – with some functionality on the plugins roadmap – so I know this feature is desired by the developer and will probably be included in their next update through WP plugins.

When updating the plugin with my additions, do I also update the readme.txt file etc and add my username as a contributor? Or should I contact them and ask to be put on?

Thanks for any advice 😉