This FreshRSS extension allows you to add GitHub feeds.
GitHub has Atom feeds that require you to specify the application/atom+xml type in the Accept header. By default, when trying to add a feed to FreshRSS, it uses the following Accept header:
application/atom+xml, application/rss+xml, application/rdf+xml;q=0.9, application/xml;q=0.8, text/xml;q=0.8, text/html;q=0.7, unknown/unknown;q=0.1, application/unknown;q=0.1, */*;q=0.1
...and GitHub doesn't seem to like that, and won't send a valid Atom feed. This extension overrides that for feeds from github.com, allowing you to add those feeds to FreshRSS. That's all.