Improve the packer docs.
Improve the packer docs.

Testing Done:
Rendered here:
  https://github.com/jsirois/pants/blob/jsirois/packer/improve_docs/build-support/packer/README.md

CI went green here:
  http://jenkins.pantsbuild.org/job/pantsbuild/job/pants/branch/PR-3350/5/

Bugs closed: 3340, 3350

Reviewed at https://rbcommons.com/s/twitter/r/3834/

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