docs: clarify nullable Channel link

README.md

@@ -50,14 +50,18 @@ All channel's [required elements](https://github.com/Podcast-Standards-Project/P
 
 ```php
 $channel->title; // string
-$channel->link; // string
+$channel->link; // ?string
 $channel->description; // string
 $channel->language; // string
 $channel->image; // string
 $channel->categories; // \PhanAn\Poddle\Values\CategoryCollection<\PhanAn\Poddle\Values\Category>
 $channel->explicit; // bool
 ```
 
+> [!NOTE]
+> Although required by the standard, `link` isn't supplied by all feeds, including some bigger players like Spotify's [Megaphone](https://megaphone.spotify.com/).
+> As such, `link` is nullable in Poddle.
+
 All channel’s [recommended elements](https://github.com/Podcast-Standards-Project/PSP-1-Podcast-RSS-Specification#recommended-channel-elements) are available via the `metadata` property:
 
 ```php