message: hello world and installation readme fixes
diff: ```jsx static
  import React from 'react';
  import ReactDOM from 'react-dom';
  -import { View, Panel, PanelHeader, Group, List, Cell } from '@vkontakte/vkui';
  +import { View, Panel, PanelHeader, Header, Group, Cell } from '@vkontakte/vkui';
  import '@vkontakte/vkui/dist/vkui.css';
  function App () {
  @@ -9,11 +9,9 @@ function App () {
  <View activePanel="main">
  <Panel id="main">
  <PanelHeader>VKUI</PanelHeader>
  - <Group title="Items">
  - <List>
  + <Group header={<Header mode="secondary">Items</Header>}>
  <Cell>Hello</Cell>
  <Cell>World</Cell>
  - </List>
  </Group>
  </Panel>
  </View>
  