Add git repo address to unstable book

tesuji · web-flow · commit e39f542c5529 · 2020-04-01T19:10:19.000+07:00
diff --git a/src/doc/unstable-book/book.toml b/src/doc/unstable-book/book.toml
@@ -1,3 +1,6 @@
 [book]
 title = "The Rust Unstable Book"
 author = "The Rust Community"
+
+[output.html]
+git-repository-url = "https://github.com/rust-lang/rust/tree/master/src/doc/unstable-book"
