Add crates.io metadata to Cargo.toml.

Cargo.toml

@@ -5,3 +5,9 @@ version = "0.0.2"
 authors = ["The Rust Project Developers"]
 license = "MIT/Apache-2.0"
 homepage = "https://github.com/rust-lang/num"
+repository = "https://github.com/rust-lang/num"
+
+description = """
+Simple numerics. This crate contains basic arbitrary-sized integer,
+rational, and complex types.
+"""