Updated README with a migration example for allowing nills

If you use the migration helper for an allow_nil field, it takes a bit of digging to work out how to do it. The normal approach of `null: false` doesn't work.

I've added a working example to the section on allowing nils to make it a bit clearer:

```ruby
add_money :products, :optional_price, amount: { null: true, default: nil }
```