P. Chandra

Adds copyright notice at end of articles in full text RSS feeds, with back links to the blog and original article.
A notice gets attached at the bottom of feed articles like this 
Original Article: Title of Article (linked to permalink) 
Copyright Year. Blog Title (linked). All Rights Reserved
Your Blog title is linked to your blog Url. Your article title is linked to the article permalink.
Blog copyright notices are important as your feed gets freely distributed across the web and is republished without permission by RSS scrappers and autoblogs. After using this plugin, now RSS scrappers will link back to your site, which will suggest to search engines that your blog and article was the original source.

Simple Feed Copyright

Adds copyright notice at end of articles in full text RSS feeds, with back links to the blog and original article.

Matthias Pfefferle

ActivityStreams (<a href="http://www.activitystrea.ms" rel="nofollow ugc">activitystrea.ms</a>) support for your WordPress-blog
ActivityStreams is an open format specification for activity stream protocols, which are used to syndicate activities taken in social web applications and services, similar to those in Facebook’s Newsfeed, FriendFeed, the Movable Type Action Streams plugin, etc.

ActivityStream extension

ActivityStrea.ms feeds for WordPress (Atom and JSON(-LD))

Simple Feed Copyright vs ActivityStream extension