Update reference.md
Add section for range expressions.
@@ -2806,6 +2806,33 @@ _panicked state_.
(["a", "b"])[10]; // panics
```
+### Range expressions
+
+```{.ebnf .gram}
+range_expr : expr ".." expr |
+ expr ".." |
+ ".." expr |
+ ".." ;
+```
+
+The `..` operator will construct an object of one of the `std::ops::Range` variants.
+
+```
+1..2; // std::ops::Range
+3..; // std::ops::RangeFrom
+..4; // std::ops::RangeTo
+..; // std::ops::RangeFull
+```
+
+The following expressions are equivalent.
+
+```
+let x = std::ops::Range {start: 0, end: 10};
+let y = 0..10;
+
+assert_eq!(x,y);
+```
+
### Unary operator expressions
Rust defines three unary operators. They are all written as prefix operators,
