How do I inform visitors that I have RSS feed?

The easiest way is to create a link to RSS-feed. By clicking it users will be able to subscribe for updates via their RSS-aggregator.
  <a href="URL**">Subscribe via RSS</a>

Another way is to add a following meta-tag to the <head> section of the HTML-page:
  <link rel="alternate" type="application/rss+xml" title="Feed Title*" href="URL**">

* "Feed Title" - RSS feed title,
** "URL" - Destination feed URL.
If you have this tag, some browsers add a special icon, which is a link to your RSS feed, located in the right corner of the address bar.