Link to references section when they first appear

In a straight-through read of "Syntax and Semantics," the concept of a
"reference" is used here before it is explained. Mention that and link to
the section explaining references.

src/doc/book/primitive-types.md

@@ -167,8 +167,11 @@ variable binding. Slices have a defined length, can be mutable or immutable.
 ## Slicing syntax
 
 You can use a combo of `&` and `[]` to create a slice from various things. The
-`&` indicates that slices are similar to references, and the `[]`s, with a
-range, let you define the length of the slice:
+`&` indicates that slices are similar to [references][], which we will cover in
+detail later in this section. The `[]`s, with a range, let you define the
+length of the slice:
+
+[references]: references-and-borrowing.html
 
 ```rust
 let a = [0, 1, 2, 3, 4];