update tutorial

docs/tutorial.md

@@ -60,3 +60,15 @@ Once an action is defined, its creator is accessible via the `actions` accessor.
 console.log( todosDux.actions.addTodo('write tutorial') ); 
     // prints { type: 'addTodo', payload: 'write tutorial' }
 ```
+### Adding a mutation
+
+Mutations are the reducing functions associated to actions. They
+are defined via the `setMutation` method:
+
+
+    ```js
+todosDux.setMutation( 'addTodo', description => ({next_id: id, todos}) => ({
+    next_id: 1 + id,
+    todos: [...todos, { description, id, done: false }]
+}));
+```