docs(readme): record the MAC-less discovery contract (t37 D3, t39)

t37 made MAC-less zeroconf discovery honour Home Assistant's own
`_async_handle_discovery_without_unique_id` contract — the helper is a coroutine
and its dedup (abort `already_configured` when ANY entry exists for this domain,
abort `already_in_progress` when another flow runs) only happens when it is
awaited. That is stricter than the accidental host-equality guard it replaced,
so it changes what a user can expect:

* with a MAC, discovery still aborts duplicates and updates a moved device's
  hosts;
* without a MAC, a second (or third) Shield must be added MANUALLY — discovery
  will abort as "already configured".

Someone with two Shields would otherwise read that abort as a bug. The README
now has a "Discovery (zeroconf)" section under Configuration stating the rule and
the manual route, and the drift sweep's docstring carries the same note next to
the D3 entry, so the decision is discoverable from wherever you meet it.

Verified: 289 passed, 2 skipped; ruff clean.

README.md

@@ -182,6 +182,24 @@ device page in HA.
 
 Requirements: Settings > Developer options > Network debugging enabled on the Shield.
 
+### Discovery (zeroconf)
+
+Home Assistant can discover a Shield through the `_androidtvremote2._tcp`
+service, and the discovered address is pre-filled on the setup form. The dedup
+rules are Home Assistant's own, and they differ with what the discovery carries:
+
+- **with** the device MAC, a duplicate aborts as "already configured", and an
+  existing entry's hosts are updated when the address changed;
+- **without** a MAC, the flow follows HA's stricter
+  `_async_handle_discovery_without_unique_id` contract: **any** existing entry
+  for this integration aborts the discovery, and only one discovery flow runs at
+  a time.
+
+So a second (or third) Shield that advertises no MAC has to be added **manually**
+— Settings > Devices & Services > Add Integration > "NVIDIA Shield" — rather than
+by discovery. That is a deliberate, tested contract (t37 D3), not a bug: the
+helper the flow calls is a coroutine and its dedup only runs when awaited.
+
 ### Options
 
 - **Jellyfin Media Player** — auto-detected or manually selected