Add private runner warnings

doc/ci/runners/README.md

@@ -135,6 +135,14 @@ Consider that if you don't lock your specific Runner to a specific project, any
 user with Maintainer role in you project can assign your Runner to another arbitrary
 project without requiring your authorization, so use it with caution.
 
+CAUTION: **Caution:**
+Never add a private Runner that you're using in your private projects to a
+project that you share with other people.
+
+CAUTION: **Caution:**
+Never use a Runner from a project which has multiple maintainers in your
+private project.
+
 An admin can enable/disable a specific Runner for projects:
 
 1. Navigate to **Admin > Runners**