add license and readme

LICENSE

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+MIT License
+
+Copyright (c) 2025
+
+Permission is hereby granted, free of charge, to any person obtaining a copy...
+[etc.]

README.md

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+# convert_dirs.sh
+
+Converts video files in a directory to `.mp4` using ffmpeg.
+
+Supports:
+
+- Dry-run mode
+- Archiving originals
+- Custom CRF quality
+- Filter by source file types (e.g., .avi, .mkv)
+
+## Usage
+
+\`\`\`bash
+./convert_dirs.sh --source ./tv --src-type avi --src-type mkv --quality 22 --dry-run
+\`\`\`
+
+## Requirements
+
+- bash
+- ffmpeg