Add note about default arguments

Author: ryanmcdermott

README.md

@@ -176,6 +176,10 @@ function paintCar(car) {
 **[⬆ back to top](#table-of-contents)**
 
 ### Use default arguments instead of short circuiting or conditionals
+Default arguments are often cleaner than short circuiting. Be aware that if you
+use them, your function will only provide default values for `undefined`
+arguments. Other "falsy" values such as `''`, `""`, `false`, `null`, `0`, and
+`NaN`, will not be replaced by a default value.
 
 **Bad:**
 ```javascript