Updated readme: Adding section on var versus let/const

Author: DrkSephy

README.md

@@ -21,3 +21,25 @@ function getFood(food) {
 
 getFood(false); // undefined
 ```
+
+However, observe what happens when we replace **var** using **let**:
+
+```javascript
+let snack = 'Meow Mix';
+
+function getFood(food) {
+
+    if (food) {
+        let snack = 'Friskies';
+        return snack;
+    }
+    return snack;
+}
+
+getFood(false); // 'Meow Mix'
+```
+
+This bug highlights that we need to be careful when refactoring legacy code which uses **var**. 
+
+> **Best Practice**: Leave **var** declarations inside of legacy code to denote that it needs to be carefully refactored. When working on a new codebase, use **let** for variables that will change their value over time, and **const** for variables that will be immutable over time.
+