Set Emacs 30 category in display-buffer action

* hackernews.el (hackernews--display-items): Specify hackernews
category when displaying rendered buffer.
* README.md (Customization): Give category example for
display-buffer-alist.

README.md

@@ -154,7 +154,12 @@ If you prefer to roll out your own Elisp, you could add to your
 `user-init-file` something as simple as:
 
 ```el
-(push '("\\`\\*hackernews .*\\*\\'" (display-buffer-same-window))
+(push '("\\`\\*hackernews .*\\*\\'" display-buffer-same-window)
+      display-buffer-alist)
+
+;; ...or equivalently, starting with Emacs 30:
+
+(push '((category . hackernews) display-buffer-same-window)
       display-buffer-alist)
 ```
 

hackernews.el

@@ -578,7 +578,7 @@ their respective URLs."
     ;; Persist new offset
     (setcar reg (+ (car reg) nitem)))
 
-  (pop-to-buffer (current-buffer))
+  (pop-to-buffer (current-buffer) '(() (category . hackernews)))
   (run-hooks 'hackernews-finalize-hook))
 
 ;; TODO: Derive from `tabulated-list-mode'?