Add documentation field to manifest

Add a `documentation` field to the `package` section of the manifest
pointing to `https://docs.rs/bech32`.

Author: tcharding

Cargo.toml

@@ -3,6 +3,7 @@ name = "bech32"
 version = "0.9.0"
 authors = ["Clark Moody", "The rust-bitcoin developers"]
 repository = "https://github.com/rust-bitcoin/rust-bech32"
+documentation = "https://docs.rs/bech32/"
 description = "Encodes and decodes the Bech32 format"
 readme = "README.md"
 keywords = ["base32", "encoding", "bech32"]