Add example to Snack, add project to native.directory

https://snack.expo.io can load packages from npm now :) I put the example from your README in Snack here: https://snack.expo.io/ryZFeWMYZ -- I also fixed a few missing imports.

It also might be worth adding this project to http://native.directory/ -- check this out for more info: https://github.com/react-community/native-directory#how-to-add-a-library