Clarify proper type casting for Numbers

Clarifies when to use `Number` vs. `parseInt` for Number type casting.

Fixes: airbnb#523

README.md

@@ -1640,7 +1640,7 @@ Other Style Guides
     const totalScore = String(this.reviewScore);
     ```
 
-  - [21.3](#21.3) <a name='21.3'></a> Use `parseInt` for Numbers and always with a radix for type casting.
+  - [21.3](#21.3) <a name='21.3'></a> Numbers: Use `Number` for type casting and `parseInt` always with a radix for parsing strings.
 
     ```javascript
     const inputValue = '4';