Add example with full working code that creates some Nested Set structure

Now we have a lof of examples for exact functions, but not the main "quick start" example that shows the code, that works out-of-the-box.

So I suggest to add a "Quick start" one-file example with full `index.js` file, that requires `sequelize` library, creates a connection, creates `Tag` nested set, adds some elements to it, then moves some to other branch, etc.