A nip-05 profile url, like

https://njump.me/dtonon.com

Or a npub one

https://njump.me/npub10000003zmk89narqpczy4ff6rnuht2wu05na7kpnh3mak7z2tqzsv8vwqk

Or a relay

https://njump.me/r/relay.nostr.band

If you feed reader does not auto discover the rss, add ".rss" to the npub or the relay versions:

https://njump.me/npub10000003zmk89narqpczy4ff6rnuht2wu05na7kpnh3mak7z2tqzsv8vwqk.rss

https://njump.me/r/relay.nostr.band.rss