docs: clarify where App Group instances are stored

[hirvesh]

What

Docs only, no behavior change. Adds a note to the "App Groups or Extensions" section of the README:

• If you don't pass a path, App Group instances are stored in the root of the App Group container.
• Native MMKV's MMKV.initialize(rootDir:groupDir:) and react-native-mmkv V2 use <container>/mmkv instead.
• A Swift snippet that opens the same instance from an extension (rootPath: groupDir, .multiProcess).

Why

The README says data is shared with extensions through multi-process mode, but not where it lives. An extension that follows the standard native MMKV setup (initialize(rootDir:groupDir:)) reads <container>/mmkv/<id> while the app writes <container>/<id>. Both sides work, they just never see each other's data, so it's easy to miss.

Changing the default isn't an option at this point. It would move every existing V3/V4 App Group install's data, as was pointed out on #813. So this PR documents the current behavior instead, as suggested in #813.

Related (all closed)

• feat: Support AppGroups #705: added App Group support in V3 with the container-root path
• Data Loss Migrating from V2 -> V3 with app group set on iOS #812: data loss upgrading from V2 to V3 with an App Group (V2 used <container>/mmkv)
• fix: Append mmkv to app group default path to mimick MMKV library #813: proposed appending /mmkv; closed as a breaking change, with a README note suggested instead
• [iOS] Data loss after upgrading from v2 → v3 (AppGroup auto-detection issue) #901: same V2 to V3 data loss, closed as a duplicate of Data Loss Migrating from V2 -> V3 with app group set on iOS #812
• App group enhancement introduced a breaking change (2.10.0) #627: an earlier App Group storage move in 2.10.0

We ran into this when adding iOS widgets: our widget and App Intents use initialize(rootDir:groupDir:), and we've been carrying the #813 change as a patch since V3.