Documents that JSX tool is transforming files with .js extension (and not .jsx) by default

docs/09-tooling-integration.md

@@ -32,7 +32,11 @@ If you like using JSX, we provide an in-browser JSX transformer for development
 
 ### Productionizing: Precompiled JSX
 
-If you have [npm](http://npmjs.org/), you can simply run `npm install -g react-tools` to install our command-line `jsx` tool. This tool will translate files that use JSX syntax to plain JavaScript files that can run directly in the browser. It will also watch directories for you and automatically transform files when they are changed; for example: `jsx --watch src/ build/`. Run `jsx --help` for more information on how to use this tool.
+If you have [npm](http://npmjs.org/), you can simply run `npm install -g react-tools` to install our command-line `jsx` tool. This tool will translate files that use JSX syntax to plain JavaScript files that can run directly in the browser. It will also watch directories for you and automatically transform files when they are changed; for example: `jsx --watch src/ build/`.
+
+By default JSX files with a `.js` extension are transformed. Use `jsx --extension jsx src/ build/` to transform files with a `.jsx` extension.
+
+Run `jsx --help` for more information on how to use this tool.
 
 
 ### Helpful Open-Source Projects