Add keywords to cargo metadata.

Cargo.toml

@@ -1,12 +1,12 @@
 [package]
 
 name = "num"
-version = "0.0.3"
+version = "0.0.4"
 authors = ["The Rust Project Developers"]
 license = "MIT/Apache-2.0"
 homepage = "https://github.com/rust-lang/num"
 repository = "https://github.com/rust-lang/num"
-
+keywords = ["mathematics", "numerics"]
 description = """
 Simple numerics. This crate contains basic arbitrary-sized integer,
 rational, and complex types.