docs: document RSS feed plugin

docs/USAGE.md

@@ -528,3 +528,32 @@ List:
 - Full scan sends acknowledgment before probing
 - 8 parallel workers, 20s overall timeout
 - Three check methods: HTTP status, JSON API, body search
+
+### `!rss` -- RSS/Atom Feed Subscriptions
+
+Subscribe RSS or Atom feeds to IRC channels with automatic polling and
+new-item announcements.
+
+```
+!rss add <url> [name]       Subscribe a feed to this channel (admin)
+!rss del <name>             Unsubscribe a feed (admin)
+!rss list                   List feeds in this channel
+!rss check <name>           Force-poll a feed now
+```
+
+- `add` and `del` require admin privileges
+- All subcommands must be used in a channel (not PM)
+- If `name` is omitted on `add`, it is derived from the URL hostname
+- Names must be lowercase alphanumeric + hyphens, 1-20 characters
+- Maximum 20 feeds per channel
+
+Polling and announcements:
+
+- Feeds are polled every 10 minutes by default
+- On `add`, existing items are recorded without announcing (prevents flood)
+- New items are announced as `[name] title -- link`
+- Maximum 5 items announced per poll; excess shown as `... and N more`
+- Titles are truncated to 80 characters
+- Supports HTTP conditional requests (`ETag`, `If-Modified-Since`)
+- 5 consecutive errors doubles the poll interval (max 1 hour)
+- Feed subscriptions persist across bot restarts via `bot.state`