Update readme with what files are needed.

README.md

@@ -72,15 +72,16 @@ You can also view all the test pdf files on the right side serving
 
 ## Building PDF.js
 
-In order to bundle all `src/` files into a final `pdf.js` and build the generic
+In order to bundle all `src/` files into two productions scripts and build the generic
 viewer, issue:
 
     $ node make generic
 
-This will generate the file `build/generic/build/pdf.js` that can be included in
-your final project. The pdf.js file is large and should be minified for
-production. Also, if you would like to support more browsers than Firefox you'll
-also need to include `compatibility.js` from `build/generic/web/`.
+This will generate `pdf.js` and `pdf.worker.js` in the `build/generic/build/` directory.
+Both scripts are needed but only `pdf.js` needs to be included since `pdf.worker.js` will
+be loaded by `pdf.js`. If you want to support more browsers than Firefox you'll also need
+to include `compatibility.js` from `build/generic/web/`. The PDF.js files are large and
+should be minified for production.
 
 ## Learning