dependencies should be better documented

[janstarke]

yara-sys requires some external dependencies (e.g. linux-libc-dev) which needs to be better documented, otherwise you'll get errors like

error: failed to run custom build command for `yara-sys v0.29.0`
note: To improve backtraces for build dependencies, set the CARGO_PROFILE_DEV_BUILD_OVERRIDE_DEBUG=true environment variable to enable debug information generation.

Caused by:
  process didn't exit successfully: `/home/dma/Tools/dionysos/target/debug/build/yara-sys-a5e93963b3119d06/build-script-build` (exit status: 101)
  --- stdout
  cargo:rustc-link-lib=dylib=yara
  cargo:rerun-if-env-changed=YARA_LIBRARY_PATH
  cargo:rerun-if-env-changed=YARA_LIBRARY_PATH_x86_64-unknown-linux-gnu
  cargo:rerun-if-env-changed=YARA_LIBRARY_PATH_x86_64_unknown_linux_gnu

  --- stderr
  /usr/include/stdio.h:34:10: fatal error: 'stddef.h' file not found
  thread 'main' panicked at /home/dma/.cargo/registry/src/index.crates.io-6f17d22bba15001f/yara-sys-0.29.0/build.rs:447:43:
  Unable to generate bindings: ClangDiagnostic("/usr/include/stdio.h:34:10: fatal error: 'stddef.h' file not found\n")
  stack backtrace:
     0: rust_begin_unwind
               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869/library/std/src/panicking.rs:665:5
     1: core::panicking::panic_fmt
               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869/library/core/src/panicking.rs:76:14
     2: core::result::unwrap_failed
               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869/library/core/src/result.rs:1699:5
     3: core::result::Result<T,E>::expect
     4: build_script_build::bindings::add_bindings
     5: build_script_build::main
     6: core::ops::function::FnOnce::call_once
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.