[editorial] add var info to let/const section

Author: Kelta-King

README.md

@@ -139,18 +139,22 @@ Other Style Guides
     ```
 
   <a name="references--block-scope"></a><a name="2.3"></a>
-  - [2.3](#references--block-scope) Note that both `let` and `const` are block-scoped.
+  - [2.3](#references--block-scope) Note that both `let` and `const` are block-scoped, whereas `var` is function-scoped.
 
     ```javascript
     // const and let only exist in the blocks they are defined in.
     {
       let a = 1;
       const b = 1;
+      var c = 1;
     }
     console.log(a); // ReferenceError
     console.log(b); // ReferenceError
+    console.log(c); // Prints 1
     ```
 
+    In the above code, you can see that referencing `a` and `b` will produce a ReferenceError, while `c` contains the number. This is because `a` and `b` are block scoped, while `c` is scoped to the containing function.
+
 **[⬆ back to top](#table-of-contents)**
 
 ## Objects