[Documentation] name_id_format_allow_create and metadata folder

[peppelinux]

Added name_id_format_allow_create and an example of metadata local folder.

[peppelinux]

I think that also only_use_keys_in_metadata should be documented for security reason.
See #592

Also these could be usefull to be documented:

            'validate_certificate': True,
            "want_authn_requests_only_with_valid_cert": True,
            'sign_response': True,
            'sign_assertion': True,

[peppelinux]

Hi @c00kiemon5ter, isn't this an easy PR for the project?
It's only documentation, if you see any breakables in it please let me know.

[ioparaskev]

😛