Improved guidance for new SIPNET contributors

[dlebauer]

Need clearer guidance and onboarding for first-time contributors.

• Update CONTRIBUTING
  • workflow from proposing an issue & discussion (gh or slack), through submitting and merging PR
  • Current ‘github workflow’ is too sparse and does not include expectations for a PR (e.g. motivation, etc)
  • link to good first issues
• add PR template
  • some instructions can go here, but mainly point back to contributing.