Add pull request review guidelines (#6374)

Add PR review guidelines for maintainers

Co-authored-by: Yuri Sizov <[email protected]>

Author: clayjohn

contributing/workflow/index.rst

@@ -15,4 +15,5 @@ approach the project.
    bisecting_regressions
    bug_triage_guidelines
    pr_workflow
+   pr_review_guidelines
    testing_pull_requests