docs: update feed section

docs/src/users/feed.md

@@ -22,5 +22,43 @@ These feeds should be auto-discoverable on the homepage:
 
 Hatsu detects all available feeds and prioritizes them in order of `JSON > Atom > RSS`.
 
+## JSON Feed
+
+Hatsu uses `serde` to parse JSON Feed directly, so you can expect it to have first-class support.
+
+Please make sure your feed is valid in [JSON Feed Validator](https://validator.jsonfeed.org/) first.
+
+### JSON Feed Items
+
+Hatsu infers object id from `item.url` and `item.id`.
+
+It will use the `item.url` first, and if it doesn't exist, it will try to convert the `item.id` to an absolute url.
+
+```text
+https://example.com/foo/bar => https://example.com/foo/bar
+/foo/bar => https://example.com/foo/bar 
+foo/bar => https://example.com/foo/bar
+```
+
+Ideally, your `item.id` and `item.url` should be consistent absolute links:
+
+```json
+{
+	"id": "https://example.com/foo/bar",
+    "url": "https://example.com/foo/bar",
+	"title": "...",
+	"content_html": "...",
+	"date_published": "..."
+}
+```
+
+### JSON Feed Extension
+
 If you can customize your site's JSON Feed,
 you might also want to check out the [Hatsu JSON Feed Extension](../others/json-feed-extension.md).
+
+## Atom / RSS
+
+Hatsu uses `feed-rs` to parse XML feeds and convert them manually.
+
+This section is currently lacking testing, so feel free to report bugs.