Update documentation links

Cargo.toml

@@ -6,6 +6,7 @@ authors = ["The Rust Project Developers"]
 license = "MIT/Apache-2.0"
 homepage = "https://github.com/rust-lang/num"
 repository = "https://github.com/rust-lang/num"
+documentation = "http://doc.rust-lang.org/num"
 keywords = ["mathematics", "numerics"]
 description = """
 Simple numerics. This crate contains basic arbitrary-sized integer,

README.md

@@ -2,6 +2,8 @@
 
 Arbitrary sized numeric types for Rust.
 
+[Documentation](http://doc.rust-lang.org/num)
+
 ## Usage
 
 Add this to your `Cargo.toml`: