How do I show a "read more" link in my RSS feed?
That's what Teasers are for — the Teaser is the shortened version of the content, presented in RSS feeds (among other places, such as the site's front page).

When editing a node in Drupal 6, use the "Split summary at cursor" button to split the node's body into a Summary/"Teaser" and "Full view".

Then go to your site's /admin/content/rss-publishing page, and set "Feed content" to "Title plus teaser".

Additionally, on your site's /admin/content/node-settings page, you can configure the default length to use to split nodes.

For some additional customization, try the Read More Link module (but be aware of this issue), or Display Suite, as @Decipher mentioned.