Currently CONTRIBUTING.md says that PRs that affect draft
files have 'relaxed' approval requirements but does not
specify what those relaxed requirements are.

This PR makes the default requirements for draft PRs explicit
(only 1 reviewer, no waiting period) in an attempt to speed
progress on draft changes. Approving reviewers will have
the opportunity to indicate if they want additional reviewers
prior to submitting these draft PRs.

We will still have strict requirements (3 approvers, 72h
waiting period) when moving content from draft to rc or
final status. This should allow thorough review of changes
in their finished state without hampering progress on new
tracks.

Signed-off-by: Tom Hennen <tomhennen@google.com>

Signed-off-by: Tom Hennen <tomhennen@google.com>

Signed-off-by: Tom Hennen <tomhennen@google.com>