Beginnings of commandsModule docs

docs/latest/advanced/extensions.md

@@ -91,7 +91,38 @@ and how we determine when/where it should be used is included below.
 
 #### Commands
 
-...
+The Commands Module allows us to register one or more commands scoped to
+specific contexts. Commands can be run by [hotkeys][#], [toolbar buttons][#],
+and any registered custom react component (like a [viewport][#] or [panel][#]).
+Here is a simple example commands module:
+
+```js
+{
+    getCommandsModule() {
+        return {
+            actions: {
+                speak: ({ viewports, words }) => {
+                    console.log(viewports, words);
+                },
+            },
+            definitions: {
+                rotateViewportCW: {
+                    commandFn: actions.rotateViewport,
+                    storeContexts: ['viewports'],
+                    options: { rotation: 90 }
+                },
+                rotateViewportCCW: {
+                    commandFn: actions.rotateViewport,
+                    storeContexts: ['viewports'],
+                    options: { rotation: -90 },
+                    context: 'ACTIVE_VIEWER::CORNERSTONE'
+                },
+            },
+            defaultContext: 'VIEWER'
+        }
+    }
+}
+```
 
 #### Viewport