Clarify system build requirements

[daveriedstra]

Just ran into this issue when trying to use this for the first time. It'd be good to have these dependencies mentioned in the readme.

For future seekers, you just need to install pkg-config and libudev-dev (on debian / ubuntu derivatives). Here's the relevant build output:

$ cargo build
   Compiling libudev-sys v0.1.4
   Compiling iovec v0.1.4
   Compiling bitflags v1.2.1
   Compiling uinput-sys v0.1.7
error: failed to run custom build command for `libudev-sys v0.1.4`

Caused by:
  process didn't exit successfully: `/home/dried/Software/rust-fun/uinput-test/target/debu
g/build/libudev-sys-226511dda7b0ba42/build-script-build` (exit code: 101)
--- stdout
cargo:rerun-if-env-changed=LIBUDEV_NO_PKG_CONFIG
cargo:rerun-if-env-changed=PKG_CONFIG
cargo:rerun-if-env-changed=LIBUDEV_STATIC
cargo:rerun-if-env-changed=LIBUDEV_DYNAMIC
cargo:rerun-if-env-changed=PKG_CONFIG_ALL_STATIC
cargo:rerun-if-env-changed=PKG_CONFIG_ALL_DYNAMIC
cargo:rerun-if-env-changed=PKG_CONFIG_PATH_x86_64-unknown-linux-gnu
cargo:rerun-if-env-changed=PKG_CONFIG_PATH_x86_64_unknown_linux_gnu
cargo:rerun-if-env-changed=HOST_PKG_CONFIG_PATH
cargo:rerun-if-env-changed=PKG_CONFIG_PATH
cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR_x86_64-unknown-linux-gnu
cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR_x86_64_unknown_linux_gnu
cargo:rerun-if-env-changed=HOST_PKG_CONFIG_LIBDIR
cargo:rerun-if-env-changed=PKG_CONFIG_LIBDIR
cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64-unknown-linux-gnu
cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_x86_64_unknown_linux_gnu
cargo:rerun-if-env-changed=HOST_PKG_CONFIG_SYSROOT_DIR
cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR

--- stderr
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "`\"pkg-config\" \
"--libs\" \"--cflags\" \"libudev\"` did not exit successfully: exit code: 1\n--- stderr\nP
ackage libudev was not found in the pkg-config search path.\nPerhaps you should add the di
rectory containing `libudev.pc\'\nto the PKG_CONFIG_PATH environment variable\nNo package 
\'libudev\' found\n"', /home/dried/.cargo/registry/src/github.com-1ecc6299db9ec823/libudev
-sys-0.1.4/build.rs:38:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace