added jest and a new test

playground/src/reducers/__tests__/reducers.spec.ts

@@ -0,0 +1,10 @@
+import { reducer } from '../index';
+
+describe('reducers', () => {
+  it('should add item to the list', () => {
+    const newStore = reducer({ todos: {}, filter: 'all' }, { type: 'add', label: 'hello' });
+    const keys = Object.keys(newStore.todos);
+    expect(keys.length).toBe(1);
+    expect(newStore.todos[keys[0]].label).toBe('hello');
+  });
+});