Add README for command-line barcode example

[yushulx]

Motivation

• Provide clear documentation for the command-line example so users can build, run, and validate the sample without inspecting source code.
• Explain supported input formats, run modes (single file, directory, multiple inputs, interactive), and where required resources are placed after build.

Description

• Add examples/command-line/README.md with build instructions using CMake and expected executable locations for Linux/macOS/Windows.
• Document supported file extensions (.jpg, .jpeg, .png, .bmp, .gif, .tif, .tiff, .pdf), run examples for single files, directories, and multiple inputs, and interactive mode usage.
• Describe output behavior for single files, directories, and combined inputs, and include guidance to replace the embedded trial license key in main.cpp with a production key.
• List bundled sample files (single_page.tif, multi_page.tif, multi_page.pdf, test.pdf) for quick validation.

Testing

• Ran CMake configuration to validate the example build instructions with cmake -S examples/command-line -B /tmp/cmake-command-line-build, which completed successfully.

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Codex Task