Add instructions for using starter app in nodejs-express Kafka template readme.

[josiemundi]

The current readme doesn't show the user how to interact with the starter app.js so it could be quite confusing if they are new to kafka. I have added some new instructions for producing and consuming messages and restructured it a bit.

Checklist:

• [x] Read the Code of Conduct and Contributing Guidelines.

• [x] Followed the commit message guidelines.

• [x] Stack adheres to Appsody stack structure.

Modifying an existing stack:

• [x] Updated the stack version in stack.yaml

Contributing a new stack:

• Describe how application dependencies are managed:

• Explain how Appsody file watcher is utilized:

• Describe other Appsody environment variables defined by the stack image:

• Describe any limitations and known issues:

Related Issues:

Fixes #848

[neeraj-laad]

@josiemundi Can you please remove the version change? Once that is done this can be merged.