Move README.md to .github

Per this comment: https://github.com/yt-dlp/FFmpeg-Builds/issues/72#issuecomment-3186230029 Also corrected the numbering of "[Use the correct Vulkan NULL type](https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2024-September/333751.html)", which was 2. instead of 6.