To add a feed link to the main.php of your template you have to insert something along these lines between the <head> </head> section:
<link rel="alternate" type="application/rss+xml" title="FEED TITLE" href="LINK-TO-YOUR-FEED" />
Using the action plugin method you could use the TPL_METAHEADER_OUTPUT event and modify the meta links before they get rendered (example:
http://pastebin.com/m4ccb07f).