This PR adds the concept of `-contrib` repositories to the governance
rules of Substrait. To that aim, the following changes are made:

* Mention that there is a distinction between "core" and `-contrib`
  repositories in the introduction of the Substrait project. Those
  definitions are used in what follows.
* Say that contributors can have write access to `-contrib`
  repositories. This allows people to make changes to those repositories
  without being promoted to committers.
* Clarify that committers are people who have write access to "core"
  repositories. This makes clear that having write access to a
  `-contrib` repository doesn't make someone a committer.
* Add a voting rule for `-contrib` repositories: Any contributor other
  than the proposer can vote. This is a pretty low bar but it ensures
  that one other human being with some relationship to the project
  approves the change.

Signed-off-by: Ingo Müller <ingomueller@google.com>

* Apply @jacques-n's suggestion on how to give write access to
  `-contrib` repositories.
* Apply @jacques-n's suggestion on committer rights in `-contrib`
  repositories.
* Apply @jacques-n's suggestion on voting rules related to `-contrib`
  repositories.
* Fix typo in "contributor."

Co-authored-by: Weston Pace <weston.pace@gmail.com>