Merge pull request rust-lang#332 from johannst/reference-link-to-gh

reference: add link to github repo

Author: RalfJung

reference/book.toml

@@ -3,3 +3,6 @@ authors = ["The Rust Project Developers"]
 multilingual = false
 src = "src"
 title = "Unsafe Code Guidelines Reference"
+
+[output.html]
+git-repository-url = "https://github.com/rust-lang/unsafe-code-guidelines"