Review and revise README template contents

[jmuk]

As suggested by @omaray, the README contents for gax (gapic) packages would require a few minor changes.

The following items are in my mind:

• only single auth example (i.e. gcloud beta auth application-default login only)
• sample code of how to load the module
• "Next Steps" section at the end, which links to the service classes at the end

Cc: @swcloud, @bjwatson