Update reference.md

Add section for range expressions.

Author: tynopex

src/doc/reference.md

@@ -2812,6 +2812,33 @@ _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,