Update doc to clarify OWNERS requirements

Clarify that owners of seldom-updated third_party directories
must still be committers.

Change-Id: Ie2903288714d549b407194ee8818e225dd0103a1
Reviewed-on: https://chromium-review.googlesource.com/754381
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Aaron Gable <[email protected]>
Cr-Commit-Position: refs/heads/master@{#514543}
1 file changed