Include instructions on how to build the Flatpak with custom API credentials

[skutter-de]

I'm trying (and currently) failing to build the flatpak with custom API credentials, instructions in the README would certainly be useful.

Note: it seems like the flatpak-builder can't find libclang.so, any ideas?

  cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS

  --- stderr
  thread 'main' panicked at 'Unable to find libclang: "couldn't find any valid shared libraries matching: ['libclang.so', 'libclang-*.so', 'libclang.so.*', 'libclang-*.so.*'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"', /run/build/paper-plane/vendor/bindgen/lib.rs:603:31
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace