Enforce referential integrity data-model example

I was trying to create my data models following the documentation about associations but it didn't work for me. I was having problems with Sequelize creating foreign key constraints. While sequelize was creating the right fields for the right table, it didn't create the foreign key constraints (at database level). 

Thanks to @durango who pointed me in the right direction, I was able to accomplish the task. This is an example in which I used the following definition not mentioned in the docs:
```
.define('Table', {
    field_id: {
        type: DataTypes.INTEGER,
        references: "FK_TableName",
        referencesKey: 'id'
    }
})
```
I hope that this help someone who is learning Sequelize