Feature/document timestamp map

[mohithshuka]

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[mohithshuka]

Description

This PR documents the existing --timestamp-map command-line option for WebVTT output that was previously undocumented.

What this PR does

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

Background

The --timestamp-map functionality was already implemented in the codebase (as seen in src/lib_ccx/ccx_common_option.c and src/lib_ccx/ccx_encoders_webvtt.c), but was not documented in the README. This led to confusion and the long-standing issue #1127.

Testing

• Documentation-only change
• No functional changes to code
• Verified option exists and works as described

Related Issues

Fixes #1127