add description how to launch as rack app.

Hi, ianneub.

Thank you for your convenient gem, I like it.

I wanted to use your kibana-sinatra gem as rack app, but not described on the gem's README.

So I added some description about "How To Launch as Rack app" for others people like me.

(Real example files is [here](https://github.com/hirakiuc/sample-app-with-kibana-sinatra), this example need bundler.)