add unstable book to the bookshelf

Author: steveklabnik

src/doc/index.md

@@ -28,6 +28,7 @@ Rust provides a number of book-length sets of documentation, collectively
 nicknamed 'The Rust Bookshelf.'
 
 * [The Rust Programming Language][book] teaches you how to program in Rust.
+* [The Unstable Book][unstable-book] has documentation for unstable features.
 * [The Rustonomicon][nomicon] is your guidebook to the dark arts of unsafe Rust.
 * [The Reference][ref] is not a formal spec, but is more detailed and comprehensive than the book.
 
@@ -42,6 +43,7 @@ landed before then. That work is being tracked [here][38643].
 [ref]: reference/index.html
 [38643]: https://github.com/rust-lang/rust/issues/38643
 [err]: error-index.html
-[book]: book/index.html
+[unstable-book]: book/index.html
 [nomicon]: nomicon/index.html
+[unstable-book]: unstable-book/index.html