Add an RSS bot (for making announcements, for example)

For example, a French community around the Darktable software can’t set up a webhook for its Git repository, so it has no way to get a bot to post an announcement whenever a new version is released.

Added a simple bot that can follow an RSS feed and post updates, such as new software releases. In my example, it’s Darktable, using the RSS feed provided by GitHub (https://github.com/darktable-org/darktable/releases.atom). This could also be used for plenty of other things, such as blogs (especially static websites).