WordPress comes with a Global setting in Settings->Reading which allows you to switch between Full Content and Excerpt only for the RSS feeds across the site, but sometimes you need to override this setting or have both options available.
This is a simple and very light weight WordPress plugin which adds an additional parameter
feed_content to RSS feeds which controls if full post content is included in the feed.
Use http://yourWebSite/feed/?feed_content=1 URL for RSS feed with full post content, and http://yourWebSite/feed/?feed_content=0 for RSS feed with excerpt only. Your default RSS feed at http://yourWebSite/feed/ without extra parameter will use your default setting from WP Admin.