Clarify documentation for platform-specific plugins directory

[oxr463]

• [ ] https://github.com/aad-for-linux/openvpn-auth-aad#configuration
• [ ] https://github.com/aad-for-linux/openvpn-auth-aad/blob/master/doc/index.md#configuration
• [ ] https://github.com/aad-for-linux/openvpn-auth-aad/blob/master/debian/rules#L24

[oxr463]

The documentation will live in the aad-for-linux repository.