Docs: add a simple usage example for npmInstall

Hey guys,

thank you for writing yeoman. When I started to understand how the parameters from npmInstall have to look like, I felt it could need a small usage example in the docu.

Questions:
  - should it be called with this.on('end') per default? (is yeoman doing this in the current version automatically?
  - should this example be improved in some ways?
  - do you need a CLA from me? (I'm totally fine that you use all my contributions)
  - may I improve the api docs as well?

best regards
Philipp