This adds the alternative outputs as tags in the <head>, enabling
the inclusion of RSS, ATOM and othe types of output types.
For example, with:
[outputs]
home = ["HTML", "RSS"]
This will now result in outputting a <link rel="alternate"
type="application/rss+xml" href="...index.xml" title="..."/>
It's implemented as described on: https://gohugo.io/templates/rss/.