Adding demographic metadata (e.g., geo)

[ekr]

The text says:

The oblivious proxy resource MUST NOT add
information about the client to this request.

This seems perhaps too strong. One might imagine wanting to for instance indicate the geographic location of the client at some broad granularity.

More generally, the client depends on correct proxy behavior and obviously this MUST doesn't guarantee actual compliance, so perhaps we should just remove the MUST in favor of a description of the implications of various choices. See also issue #66