clarify polyfill usage

doc/polyfill.md

@@ -12,6 +12,9 @@ when using [6to5-node](usage.md#node).
 
 ### Node
 
+You need to include the polyfill require at the top the **entry point** to your
+application.
+
 ```javascript
 require("6to5/polyfill");
 ```
@@ -19,4 +22,6 @@ require("6to5/polyfill");
 ### Browser
 
 Available from the `browser-polyfill.js` file within the 6to5 directory of an
-npm release.
+npm release. This needs to be included **before** all your compiled 6to5 code.
+You can either prepend it to your compiled code or include it in a `<script>`
+before it.