Package config is also a dependency on Linux

[tversteeg]

It might be useful to add it to the README.
If it's missing it throws the following error:

error: failed to run custom build command for `x11 v2.17.5`
process didn't exit successfully: `/home/thomas/wtftw/target/release/build/x11-40a5afbe4e71f86e/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Command { command: "\"pkg-config\" \"--libs\" \"--cflags\" \"x11 >= 1.4.99.1\"", cause: Os { code: 2, kind: NotFound, message: "No such file or directory" } }', libcore/result.rs:945:5
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:475
   5: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   6: rust_begin_unwind
             at libstd/panicking.rs:325
   7: core::panicking::panic_fmt
             at libcore/panicking.rs:77
   8: core::result::unwrap_failed
   9: build_script_build::main
  10: std::rt::lang_start::{{closure}}
  11: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  12: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105
  13: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  14: main
  15: __libc_start_main
  16: _start

warning: build failed, waiting for other jobs to finish...
error: build failed```