Add cargo.toml metadata to link to crate documentation

Without this, crates.io does not put a Documentation link on the search page.

Author: dtolnay

Cargo.toml

@@ -4,6 +4,7 @@ version = "1.0.14"
 authors = ["David Tolnay <[email protected]>"]
 categories = ["development-tools", "no-std", "no-std::no-alloc"]
 description = "Macros for all your token pasting needs"
+documentation = "https://docs.rs/paste"
 edition = "2018"
 keywords = ["macros"]
 license = "MIT OR Apache-2.0"