Policy Model - 1st para

This whole paragraph suffers from the lack of context (due to removal of text to the XDI connections?). We should rework it to allow for a more progressive introduction of components / concepts. For instance, we could explain that:

1. by default, a graph is not shared with anyone
2. Link contracts govern the sharing of a graph
3. Sharing may be anonymous/public (i.e. accessible to anyone) - done via a public LC
4. Sharing may be private - done via generic or specific LC - point to XDI connection spec
5. last paragraph on graph ownership & control (also: should vs. SHOULD in the last sentence)