Stage 4: Document async/await + optional UniTask integration

[MaxHeimbrock]

Summary

Capstone of the UniTask migration. Adds a README section, "Asynchronous programming: coroutines, async/await, and UniTask", documenting the three interchangeable async styles the SDK now supports, and states the forward policy.

Policy (decided): keep both, no deprecation. Coroutines (yield return) remain the default and fully supported; async/await and UniTask are purely additive opt-ins. The coroutine API is not deprecated and no removal is planned — any future change would go through a deprecation cycle.

What the new section documents

1. Coroutines — the default (already shown throughout the README).
2. async/await, no dependency — every instruction is awaitable (Stage 1 GetAwaiter); inspect IsError on the instruction, await does not throw (parity with yield return); continuations resume on the main thread. Includes the async void caveat for event handlers.
3. UniTask, optional — install com.cysharp.unitask; the LIVEKIT_UNITASK define auto-enables the LiveKit.UniTask assembly. Examples: AsUniTask(cancellationToken) (Stage 2), UniTask.WhenAll(...) parallel publish, and await foreach over reader.ReadIncremental().AsAsyncEnumerable(ct) with the StreamError throw-on-error contract (Stage 3). Points to the Meet sample (UniTask) and Basic sample (coroutines).

Notes

• Docs-only: one file (README.md, +56 lines). No code, no [Obsolete], no sample/scene changes, no version bump (releases use GitHub-generated notes).
• Snippets use the verified public signatures: Connect(url, token, RoomOptions), PublishTrack(track, options), ReadIncremental().AsAsyncEnumerable(). (The README's pre-existing 2-arg Connect example is stale and left untouched as out of scope.)

Targeting

Base branch is max/yield-instruction-async-enumerable (Stage 3, PR #297) so the documented API matches the full shipped surface. Chain: Stage 1 (#289) → Stage 2 (#290) → Stage 3 (#297) → Stage 4 (this).

Test plan

• Snippets cross-checked against the public API in Runtime/Scripts/ (Connect/PublishTrack overloads, AsUniTask, AsAsyncEnumerable, StreamError).
• Reviewer: eyeball the rendered README section on GitHub.

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