CONTRIBUTING.md: Change upstream policy, inactive tickets, response time
    
    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.