Added proguard rules allowing consumers of the sdk to minify their app.

[hauthorn]

When consuming this fork through the package you provided, you need some proguard rules in order to use minifyEnabled.

I've added the rules in the example project, as well as in your readme, just above the license.

I hope this can help future consumers of your work!

This is my first contribution here. Please let me know if there's something I should change before you can approve this.