How do I edit RSS-feed?

Hi

I want to add the following to the end of each post in my RSS-feed

“Read the whole post at [link]”

where [link] should (of course) be the link to the post.

But I’m not sure what file to edit.

I’ve found this, and I know it’s somewhere in /wp-includes/ – probably feed-rss2.php

But exactly here should I add the text and link I want in my RSS feed?

I only use the automatically generated excerpt of my posts in my RSS-feeds.

Can anyone help?

Thanks :o)