Go down to one ACK

Move down to one ACK rather than two for code changes. One or none for
doc changes.

CONTRIBUTING.md

@@ -21,7 +21,7 @@ Contact one of the [OWNERS](OWNERS) on Slack to access the appropriate CLA and i
 4. Fork the desired repo, develop and test your code changes.
 5. Submit a pull request.
 
-Note: Code-related PR's require two ACK's / LGTM's from a maintainer while doc-related PR's require only one
+Note: Code-related PR's require one ACK / LGTM from a maintainer or core contributor. Doc-related PR's require either one or none depending on the content changed (for example, a spec change would require one, but a spelling error would require none).
 
 ### Adding dependencies