This commit makes three key changes to the project contributing
guidelines, per discussion with @jflory7 and @jonatoni for #149:

• Upstream: Issues > PRs
  • Upstream commented in one of our applicant's PRs that they prefer
    to have issues first before PRs, so they have a chance to figure
    out if they want one of our changes. Makes sense. This is now
    codified in our docs.
• Freeing up inactive tickets
  • After 5 days of no follow-up in an assigned ticket, a ticket can
    be re-assigned by a project maintainer.
• Clarify maintainer / mentor response time
  • Mentors are committed to no more than three days turnaround on
    replies to tickets and/or pull requests.