The full rssCloud walkthrough explains how to write a cloud server or a cloud-aware aggregator. Luckily it's much simpler to publish a cloud-aware feed, which is good because there are so many of them.
If your publishing software supports rssCloud, you may only have to turn it on. You may find that it is already on, in which case you have to do nothing. ;->
All wordpress.com blogs are rssCloud-aware.
Even if your publishing software doesn't support it directly, it's easy to add it yourself.
Add a <cloud> element to near the top of your feed.
You're welcome to use my cloud server. If you do you can just copy the cloud element from my feed. But be aware that my server will go away someday. When that happens, you'll have to find another server. But hopefully by then there will be many to choose from.
You may have to write a script to do this part.
My cloud server supports the REST form of pings, as I think most of them will.
To tell my server that your feed has changed make a POST call to:
Include one parameter in the body of the post, url. Its value is the address of the feed that changed.
Once it's verified that your feed has changed, it will then ping all subscribers who have requested notification when your feed has changed.
Congratulations -- you're now doing Real-time RSS! ";->"