Add the app 'clock' to the project README.md

As you developed four apps in the path snsapi/app/, only three of them are involved in the project README.md, why not add the app 'clock'?? It is a simple and clear example, too.

By the way, you needn't wirte the .md file like [http://...](http://...), just http://... in the .md and the Markdown will consider it as an URL.