docs: add docs for augmenting PeerContext type (#161)

Author: luc122c

docs/1.guide/3.peer.md

@@ -39,6 +39,16 @@ Direct access to the [`WebSocket`](https://developer.mozilla.org/en-US/docs/Web/
 
 The context is an object that contains arbitrary information about the request.
 
+You can augment the `PeerContext` interface types to add your properties.
+
+```ts
+declare module "crossws" {
+  interface PeerContext {
+    customData?: string[];
+  }
+}
+```
+
 > [!NOTE]
 > context data can be volatile in some runtimes.