Added class diagram

To hopefully improve the understanding of the SHACL syntax and the 
relationship between classes and shapes, I have added a class diagram to 
the introduction of section 2. This is explicitly called non-normative 
and serves purely for illustration purposes, and as a navigation aid.

Please let me know if any of you see problems with such a diagram, or 
specific mistakes in the diagram.

Screenshot of current version attached.

Holger