doc: mention moderation repo in onboarding doc

Author: addaleax

doc/onboarding.md

@@ -71,6 +71,9 @@ onboarding session.
   * [**See "Who to CC in issues"**](./onboarding-extras.md#who-to-cc-in-issues)
     * will also come more naturally over time
 
+  * When a discussion gets heated, you can request that other collaborators keep an eye on it by opening an issue at the [nodejs/moderation](https://github.com/nodejs/moderation) repository.
+    * You can find the full moderation policy [here](https://github.com/nodejs/TSC/blob/master/Moderation-Policy.md).
+
 ## Reviewing PRs
   * The primary goal is for the codebase to improve.
   * Secondary (but not far off) is for the person submitting code to succeed.