Add wording for private extensions and namespaces

[martinweismann]

Something along the lines of:

The specifications of private namespaces (i.e. that are not ratified by the 3MF Consortium) need to be negotiated between parties in the ecosystem.

[martinweismann]

I will add that here https://github.com/3MFConsortium/spec_core/blob/master/3MF%20Core%20Specification.md#about-this-specification or somewhere else, if there is a better spot.