Add nix-ld dependencies for running appImage to README

[Perodactyl]

I had to rebuild my config multiple times to make the appImage run. It needed harfbuzz, fribidi, libthai, and libgpg-error. Upon successful launch I immediately got these logs:

libp11-kit.so.0: cannot open shared object file: No such file or directory
Failed to load module: /tmp/.mount_CSSLoapYUS7L/usr/lib/x86_64-linux-gnu/gio/modules/libgiognutls.so
Could not determine the accessibility bus address
Fontconfig warning: using without calling FcInit()
libp11-kit.so.0: cannot open shared object file: No such file or directory
Failed to load module: /tmp/.mount_CSSLoapYUS7L/usr/lib/x86_64-linux-gnu/gio/modules/libgiognutls.so
libp11-kit.so.0: cannot open shared object file: No such file or directory
Failed to load module: /tmp/.mount_CSSLoapYUS7L/usr/lib/x86_64-linux-gnu/gio/modules/libgiognutls.so

Also, while merging the deck documentation with the windows documentation in my head kind of works, please add install instructions for linux. I don't know how to make the backend run, either. I just want to edit my css on linux as a test and then apply it on my deck.