chore: Add an async support section to the README

[jsonbailey]

What

Adds user-facing docs for the experimental async client, plus a small PR-template aid.

README — new "Async support" section:

• Experimental caution (matches the .. caution:: blocks on the async API).
• The redis>=4.2.0 requirement when using Redis with the async client (big segments or persistent store).
• Install via the [async] extra, and a minimal AsyncLDClient usage example.
• A Deployment note: AsyncLDClient is single-event-loop / not thread-safe; construct once (its __init__ needs no running loop), then await start() per worker loop — or use async with — which fits ASGI lifespan startup.

PR template: adds a "sync/async parity" checkbox so a change to a sync file prompts a matching async update (the drop-codegen sibling-file convention).

Why

The async implementation is fully merged and importable (from ldclient import AsyncLDClient, [async] extra), but the README had no async entry point. This is the last user-facing gap before an experimental release.

Both files are Markdown, so the test/contract CI is intentionally skipped (paths-ignore: **.md) — there's nothing to test.

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Note

Overview
Documents the experimental AsyncLDClient in the README so asyncio users have a clear entry point before an experimental release.

The new Async support section adds production warnings, the launchdarkly-server-sdk[async] install path, a minimal usage example, Redis >=4.2.0 guidance for async Redis, and Event loop notes (single-loop, not thread-safe, construct without a running loop then await start() or async with, ASGI lifespan pattern).

The pull request template gains a sync/async parity checkbox so contributors remember matching async_* siblings when changing sync APIs.

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