The code is hosted on GitHub (https://github.com/lukaramishvili/cl-rss-gen).
cl-rss-gen depends on cl-who by Edi Weitz, and thanks to him cl-rss-gen is so short. You should load cl-who by your favorite package manager before loading cl-rss-gen. It's (asdf:operate 'asdf:load-op 'cl-who) for ASDF, and (ql:quickload :cl-who) for Quicklisp.
cl-rss-gen is in draft stage. I needed and couldn't find such functionality, so wrote my own. I suggest looking at the code (it's really tiny) before using it.
Anyway, for simple needs cl-rss-gen already works. Feel free to send suggestions. corrections, patches and bug-fixes to email@example.com. I will add your name to this page if the changes get incorporated (very likely so).
This library is written by Luka Ramishvili (luka.ge), in the hope that it will be useful.