documentation misses to mention required dependency

[8de2fdb0]

Installation on linux fails

error: failed to run custom build command for `foundations v3.2.2`

Caused by:
  process didn't exit successfully: `/workspaces/gateway/target/debug/build/foundations-820afc6700be98da/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-changed=build.rs
  cargo:rerun-if-changed=src/security/libseccomp
  cargo:rustc-link-lib=static=seccomp
  cargo:warning=Compiler version doesn't include clang or GCC: "cc" "--version"
  TARGET = Some("aarch64-unknown-linux-gnu")
  OPT_LEVEL = Some("0")
  HOST = Some("aarch64-unknown-linux-gnu")
  cargo:rerun-if-env-changed=CC_aarch64-unknown-linux-gnu
  CC_aarch64-unknown-linux-gnu = None
  cargo:rerun-if-env-changed=CC_aarch64_unknown_linux_gnu
  CC_aarch64_unknown_linux_gnu = None
  cargo:rerun-if-env-changed=HOST_CC
  HOST_CC = None
  cargo:rerun-if-env-changed=CC
  CC = None
  cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
  cargo:warning=Compiler version doesn't include clang or GCC: "cc" "--version"
  cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some("true")
  CARGO_CFG_TARGET_FEATURE = Some("neon")
  cargo:rerun-if-env-changed=CFLAGS_aarch64-unknown-linux-gnu
  CFLAGS_aarch64-unknown-linux-gnu = None
  cargo:rerun-if-env-changed=CFLAGS_aarch64_unknown_linux_gnu
  CFLAGS_aarch64_unknown_linux_gnu = None
  cargo:rerun-if-env-changed=HOST_CFLAGS
  HOST_CFLAGS = None
  cargo:rerun-if-env-changed=CFLAGS
  CFLAGS = None
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  exit status: 0
  cargo:rerun-if-env-changed=AR_aarch64-unknown-linux-gnu
  AR_aarch64-unknown-linux-gnu = None
  cargo:rerun-if-env-changed=AR_aarch64_unknown_linux_gnu
  AR_aarch64_unknown_linux_gnu = None
  cargo:rerun-if-env-changed=HOST_AR
  HOST_AR = None
  cargo:rerun-if-env-changed=AR
  AR = None
  cargo:rerun-if-env-changed=ARFLAGS_aarch64-unknown-linux-gnu
  ARFLAGS_aarch64-unknown-linux-gnu = None
  cargo:rerun-if-env-changed=ARFLAGS_aarch64_unknown_linux_gnu
  ARFLAGS_aarch64_unknown_linux_gnu = None
  cargo:rerun-if-env-changed=HOST_ARFLAGS
  HOST_ARFLAGS = None
  cargo:rerun-if-env-changed=ARFLAGS
  ARFLAGS = None
  cargo:rustc-link-lib=static=seccomp
  cargo:rustc-link-search=native=/workspaces/gateway/target/debug/build/foundations-36aa857ef7d35cd1/out
  cargo:rerun-if-env-changed=TARGET
  cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS_aarch64-unknown-linux-gnu
  cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS_aarch64_unknown_linux_gnu
  cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS

  --- stderr
  thread 'main' panicked at /usr/local/cargo/registry/src/index.crates.io-6f17d22bba15001f/bindgen-0.68.1/lib.rs:611:31:
  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: [])"

Can be resolved by installing libclang-dev.

apt-get install libclang-dev

Might be worth mentioning platform dependencies in the README.