Update contributing.md to include description of how to find build logs (#800)

Update contributing.md to include description of how to find build logs

Author: gongmax

contributing.md

@@ -30,3 +30,13 @@ Guidelines](https://opensource.google/conduct/).
 All submissions, including submissions by project members, require review. We 
 use [GitHub pull requests](https://docs.github.com/articles/about-pull-requests)
 for this purpose.
+
+After creating a pull request, a collaborator needs to add `/gcbrun` as a comment
+to trigger CI for the PR, which includes the CloudBuild triggered e2e test
+`pr-review-trigger` and `pr-review-trigger-ap-clusters` as shown below. 
+![image](https://github.com/user-attachments/assets/6a759456-e1dd-4a3b-95da-84c9f2b8b053)
+
+If any of the builds fail, you can find the link to the build log for these triggers at
+the bottom of the details page by clicking `Details` link. 
+
+