Adds ga-beacon to README.

[coollog]

Adds https://github.com/igrigorik/ga-beacon so we can get more in-depth traffic analytics.

Preview at: https://github.com/GoogleContainerTools/jib/tree/coollog-patch-2

[chanseokoh]

The instructions say

The ga-beacon.appspot.com instance is a demo instance, good for prototyping and proof of concepts. If you intend to use this in production for your application, you should deploy your own instance of this service,

[coollog]

I think we can add this for now to try it out (as a proof of concept/prototype), and then deploy our own instance(s) if it works well for us.

[coollog]

Will be deploying our own App Engine instance.

[coollog]

Filed #535 for moving beacon fork to team org.

The beacon is deployed and the PR is updated.

[coollog]

Made the beacon a pixel since the badge doesn't provide much value to viewers.