add plugin example for intercept

doc/plugins.md

@@ -43,6 +43,8 @@ Likewise, adding a style to `app:stylesheets` is done as follows:
 
 ## Examples
 
+### Print the selected channel
+
 Plugin that prints the name of the currently selected channel each time it changes:
 ```javascript
 // We wrap our plugin in a function that runs immediately so that we don't pollute the scope.
@@ -93,3 +95,33 @@ Plugin that prints the name of the currently selected channel each time it chang
     pluginLoaders.push(() => plugin); // window.__wafflePluginLoaders is an array of functions which return a plugin
 })();
 ```
+
+### Message signature
+
+Plugin that adds a signature after every message. This demonstrates the store.pipe() method and leaves some comments out for the sake of showcasing more ways to structure your plugin.
+Note: please don't use this in actual chats.
+
+```javascript
+(window.__wafflePluginLoaders || __wafflePluginLoaders || []).push(() => ({
+    name: "messageSignature",
+    description: "Adds a signature to every message before sending",
+    state: {
+        cleanup: []
+    },
+    options: {
+        signature: "\n\n-Waffle user"
+    },
+    main(waffle) {
+        // store.pipe() allows us to add a function to "intercept" the store value before it reaches all of its subscribers.
+        // As with store.subscribe(), store.pipe() returns a function that removes the pipe once we're done with it.
+        this.state.cleanup.push(waffle.stores.sendMessageAction.pipe(message => {
+            message.content = `${message.content}${this.options.signature}`;
+            return message;
+        }));
+    },
+    dispose(_waffle) {
+        this.state.cleanup.forEach(f => f());
+        this.state.cleanup.length = 0;
+    }
+}));
+```