Update README.md

fixes #3

README.md

@@ -25,6 +25,15 @@ Then call it by passing it a JavaScript URL:
 </head>
 ```
 
+## Why not just use `<script src="..." async defer>`?
+
+The `async` and `defer` attributes enjoy broad browser support. They're great options when all you need to do is load a script for all users, ideally in a non-blocking manner. The limitations with these attributes are: 
+
+1. Some older browsers do not support `async` ([though defer has broader support so it's typically not a problem](https://www.igvita.com/2014/05/20/script-injected-async-scripts-considered-harmful/))
+2. There's no way to use these attributes to *conditionally* load a script, depending on feature or environmental conditions. 
+
+Number 2 above is the main reason we use `loadJS`. Say you want to check if `querySelector` is supported before requesting your entire DOM framework and UI scripting - you'll need to use a script loader to do that. But again, if you just want to load a script unconditionally, these attributes are recommended.
+
 ## Limitations
 
 `loadJS` does nothing to manage execution order of requested scripts, so we do not advise using it to load multiple javascript files that depend on one another. It simply fetches a script and executes it as soon as possible. You can certainly use `loadJS` to load multiple scripts as long as those scripts are designed to execute independently of any other scripts being loaded by `loadJS`.