Add playground in the homepage

[tonyyang-svail]

The playground is now available at http://play.sqlflow.org. It is hosted on the Google Cloud Platform.

Please link it at the homepage.

[llxxxll]

国内用户访问速度比较慢，体验不是特别好。
是不是在阿里云上也部署完成后，再暴露这个入口。