chore: add note about new permissions requirement for custom domains

apps/docs/docs/guides/platform/custom-domains.mdx

@@ -19,6 +19,7 @@ To get started:
 
 1. [Install](/docs/guides/cli) the Supabase CLI 1.11.3+.
 1. [Log in](/docs/guides/cli/local-development#log-in-to-the-supabase-cli) to your Supabase account using the CLI.
+1. Ensure that you have Owner or Admin permissions for the project you'd like to set up a custom domain for.
 1. Enable the Custom Domains add-on in the Billing page of the Dashboard.
 
 ## Configure a Custom Domain