Document how to use the library

I am trying to write `extract_diricon.c` that does the following: * Extracts the file `.DirIcon` from a zstd compressed squashfs file (resolving symlinks inside the squashfs file) * The squashfs file starts at an offset of bytes, specified with `-o` Example: `./extract_diricon -o 12345 my.squashfs` should extract the file `.DirIcon`. I figure I need to compile with `gcc -o extract_diricon extract_diricon.c -I/usr/include/sqfs -L/usr/lib/x86_64-linux-gnu -lsquashfs`. But how would I have to code the above? Thank you very much for your help. (Use case: AppImage files contain a squashfs image that contains an icon, and I want to write a tiny helper to extract this icon, so that file managers can natively show the correct icons for AppImages.)