Clarifying unix-like support

Added `build.rs` to  produce a compile error when attempting to build on a
non-Unix-like system, and clarified support in `readme.md`.

Signed-off-by: Jonathan Woollett-Light <jcawl@amazon.co.uk>

build.rs

@@ -0,0 +1,4 @@
+fn main() {
+    #[cfg(not(target_family = "unix"))]
+    std::compile_error!("This crate only supports Unix-like targets");
+}