Add a tiny README.md

rust/coffi/README.md

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# coffi - Some FFI experiments in Rust
Playing with the FFI in Rust.  Surprisingly easy, but there are likely
dragons elsewhere.
## Quickstart
Display some information about a PNG file, most notably it's width and
height.
```
cargo run <path-to-png>
```
If you use emacs, `flycheck-mode` can check the code on the *fly*, which
is awesome!
## Why "coffi"?
Not sure, only the "ffi" part makes sense now.  Ah, no, the "co" comes
from `cos`, which was the first function we wanted to wrap.  That turned
out so easy that we tried `libpng` next.