reintroduce manual link in doc index

whoops

Fixes rust-lang#16827 (comment)

Author: steveklabnik

src/doc/index.md

@@ -43,6 +43,10 @@ discussion about Rust.
 There is also a [developer forum](http://discuss.rust-lang.org/), where the
 development of Rust itself is discussed.
 
+# Specification
+
+Rust does not have an exact specification, but an effort to describe as much of
+the language in as much detail as possible is in [the manual](rust.html).
 
 # Guides