Add action to generate rss feed

[phillmv]

Per rubysec/rubysec.com#14, it'd be cool and useful if we could offer folks an rss feed so they could consume the data in other ways.

I recently became adept with generating static gh-pages from actions, so should anyone want to write a script that generates an atom/rss feed given the contents of the ruby-advisory-db, I'd be happy to string that into a GitHub action.

[reedloden]

We have the RSS feed now (technically, it's Atom, but whatever), but we still need GitHub Actions to run on any commits made to ruby-advisory-db that will then run rake advisories in this repository.

[reedloden]

ok, I automated this. :-)

[jrochkind]

Thank you!