Document existing --timestamp-map option for WebVTT output

[Apprentice2907]

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Description

This pull request documents the existing --timestamp-map command-line option for WebVTT output.

The functionality to enable the X-TIMESTAMP-MAP header already exists in the codebase and is disabled by default.
However, it was not documented in the README, which led to confusion and the long-standing open issue #1127.

What this PR does

• Documents the --timestamp-map option in the README
• Explains when the X-TIMESTAMP-MAP header is needed (e.g. HLS workflows)
• Clarifies default behavior (disabled unless explicitly enabled)

What this PR does NOT do

• No functional or behavioral changes
• No new flags added
• No refactoring of encoder logic

Testing

• Documentation-only change; no runtime behavior affected

Closes #1127

[cfsmp3]

Thanks for documenting the --timestamp-map option! The PR description is clear and this addresses issue #1127.

Issue: Unclosed code block

The markdown code block is missing its closing fence:

```bash
ccextractor input.ts --timestamp-map -o output.vtt


### Windows Package Managers

This will break the README rendering. Please add the closing ``` after the example command:

```bash
ccextractor input.ts --timestamp-map -o output.vtt

Windows Package Managers

Once fixed, this is ready to merge.