Update CONTRIBUTING.md
Update CONTRIBUTING.md

- "What should I work on?" section to nudge people towards higher priority work (per @mariekers), rather than blue sky.
- Add skeleton of release workflow

Blocking:
- Now that #covid19-chime-penn is announcement-only, where should people request write access?
CodeForPhilly/chime,CodeForPhilly/chime,CodeForPhilly/chime