Make a note on Granular Permissions usage

[notrab]

If no token is provided, and permissions are added, Gatsby will fail to fetch content for the schema it has access to.

GraphCMS doesn't remove anything from introspection that you don't have access to, so Gatsby is just doing what it thinks it can with regards to the schema.

We should add a note that Gatsby needs a token with all permissions enabled to work properly.