Add note about requesting collaborator permissions

Closes https://github.com/google/iree/pull/1911

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/1911 from GMNGeoffrey:contributing f28d889c26e8b7bfd4a1126fca61ea1cf8c75e18
PiperOrigin-RevId: 311554044
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 33604e2..ef46ddc 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -34,6 +34,13 @@
 [GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
 information on using pull requests.
 
+## Presubmits
+
+Several of our presubmit builds will only run automatically if you are a project
+collaborator. Otherwise a collaborator must label the PR with "kokoro:run". If
+you are sending code changes to the project, please ask to be added as a
+collaborator, so that these can run automatically.
+
 ## Peculiarities
 
 We use a GitHub integration to import PRs into our upstream (Google internal)