README: correct the privacy claim and document what 1.2.3 added (#35)

The privacy paragraph said "your IP is never exposed to other nodes", which is
close enough to true to be relied on and not true enough to deserve it. A peer
reached over .btf genuinely does not learn your IP -- there is no IP-based
dialling left. But the rendezvous relay sees it by construction, and the node
still listens on 8433, so anyone who already knows the address can connect
directly. All three are now stated, along with the line that this is
unlinkability between peers rather than anonymity against an observer.

Also stale or missing:

- Quick start named 1.1.0 files, four releases back. Now points at the release
  page and shows how to verify SHA256SUMS.
- Nothing said upgrades matter. 1.2.1 and 1.2.2 both changed consensus, so an
  old node ends up on a different chain with no warning. Said so, with where the
  data directory lives and why not to delete it.
- Discovery was described as Nostr only. The cache and peer exchange landed in
  1.2.3; all four layers are now listed, with the 36s -> 2s restart figure.
- The glance table said "halving on schedule", which hides the number people
  actually want: same 210,000 blocks as Bitcoin, but ~292 days instead of four
  years. Added retarget window, coinbase maturity, sigops cap and P2P port too.
- /port, /datadir and /debug were undocumented, as was the fact that /gen does
  not persist across restarts.

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