Clarify the running and building process.

README.md

@@ -21,12 +21,16 @@ Clone this repo and then change directories into webxr-polyfill/
 	npm install   # downloads webpack and an http server
 	npm start     # builds the polyfill in dist/webxr-polyfill.js and start the http server in the current directory
 
-## Builds
-
-  npm run build # builds the polyfill in dist/webxr-polyfill.js
-
 Using one of the supported browsers listed below, go to http://YOUR_HOST_NAME:8080/
 
+## Portable builds
+
+To build the WebXR polyfill into a single file that you can use in a different codebase: 
+
+	npm run build
+
+The resulting file will be in dist/webxr-polyfill.js
+
 ## Writing your own XR apps
 
 The WebXR polyfill is not dependent on any external libraries, but examples/common.js has a handy base class, XRExampleBase, that wraps all of the boilerplate of starting a WebXR session and rendering into a WebGL layer using Three.js.