README: clarify streamed reader lifetime

[kumagi]

What issue is this addressing?

None

What type of issue is this addressing?

Documentation

What this PR does | solves

The streaming note suggests retaining the original file separately in a struct to prevent garbage collection. A reachable player already retains its reader, and the MP3 decoder used in this example retains the file.

Describe the actual lifetime requirement: keep the player reachable, and avoid explicitly closing the file while it is being read. Point to PauseAndStopReading for stopping reads before closing the file.

Validation

Traced Context.NewPlayer through mux.NewPlayer's retained source, and verified go-mp3's Decoder/source retains the original reader. Checked the existing NewPlayer and PauseAndStopReading contracts. No runtime behavior or example code changes.

• go test -race ./internal/... passed.
• go build ./... and git diff --check passed on macOS ARM64.
• No new tests added for the prose correction. The full native suite's existing AudioQueueStart failure / TestSetBufferSize timeout and Darwin unsafe.Pointer vet warning were previously reproduced on the unchanged upstream base; this PR does not address them.

[hajimehoshi]

LGTM