组件贡献注意事项与规范

[相关wiki](https://github.com/DFE-evernote/axe-ui/wiki/%E8%B4%A1%E7%8C%AE%E7%9B%B8%E5%85%B3%E8%A7%84%E8%8C%83%E5%92%8C%E6%B3%A8%E6%84%8F%E4%BA%8B%E9%A1%B9)

[Style规范](https://github.com/fex-team/styleguide/blob/master/css.md)