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>
The readme is still WIP, but it was added so that we can have links
to the other documents in the repository.
Signed-off-by: Andreea Florescu <fandree@amazon.com>