rust-bindings: use correct README.md

I noticed at https://crates.io/crates/ostree/0.14.0
that the `README.md` was wrong...

Cargo.toml

@@ -6,7 +6,7 @@ edition = "2018"
 keywords = ["ostree", "libostree"]
 license = "MIT"
 name = "ostree"
-readme = "README.md"
+readme = "rust-bindings/README.md"
 repository = "https://github.com/ostreedev/ostree"
 version = "0.14.0"