Update reference.mdx

Author: egelhaus

pages/configuration/reference.mdx

@@ -45,6 +45,7 @@ Refer to the [example postiz.env file](https://raw.githubusercontent.com/gitroom
 eg: `true`
 
 This will only allow a single user registration, and then after that the sign-up page will be disabled. Useful for self-hosting where you want complete control over users.
+Warning: This will disable OIDC / OAuth!
 
 ## Social Media keys