Thanks @bep but I just made it!
The solution was in the post of this thread: How to use the new Custom Output Formats - #5 by tutume
I simply named the template list.customrss
under /layouts/_default/
and BINGO! The custom RSS feed was rendered.
In my config.toml
I used exactly what you posted above.
This info is already in the docs in the table for the different Output Formats example: Custom output formats | Hugo
But it wasn’t that clear. I think I’ll send a pull request for the Docs as soon as I have more time to make this plain and simple. To include a phrase like this one:
The naming of an Output Formats template follows the same pattern that is used for standard Hugo templates. eg.
list.[kind].[suffix]
under/layouts/_default
orindex.[kind].[suffix]
under/layouts/