docs: document --allow-private-network for localhost/LAN testing in README

The flag (and OBSCURA_ALLOW_PRIVATE_NETWORK=1) were only discoverable via
--help; anyone pointing Obscura at a local dev server hits 'Access to
private/internal IP address ... is not allowed' with no hint a flag exists.

Adds a Quick Start subsection with per-invocation and env-var usage plus a
pointer to docs/Environment-variables.md for the full allow/deny rules.

Closes #695