Include README example of Underscore mixin without module loading

I was stumped for a few minutes when determining how to load this library in a project that didn't use module loading. I wanted to integrate it with Underscore, but the README only details the process when using Require.js. After looking at the source code, I see that you are creating the global variable _.str if no module loader is detected. This commit just expands the README to make this process explicit.

Thanks for an awesome library!