Create range.md

docs/range.md

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+### Range
+
+A range is an object that represents a consecutive range of numbers. Syntax for this type has been directly inspired by Swift.
+```swift
+	// a represents a range with values 1,2,3
+	var a = 1...3;
+
+	// b represents a range with values 1,2
+	var b = 1..<3;
+
+	// Ranges have also a conveniente count property
+	var n1 = a.count;	// n1 is now 3
+	var n2 = b.count;	// n2 is now 2
+```
+
+A range is expecially useful in for loops:
+```
+  for (var i in 1...10) {
+    // repeat for 10 times (with i from 1 to 10)
+  }
+```