feat: added declare to docs

[NGorco]

Adding proper model fields definitions that doesn't conflict with vanilla Sequelize.
Addressing that problem https://sequelize.org/docs/v6/core-concepts/model-basics/#caveat-with-public-class-fields

[blankstar85]

I was just reading the updated sequelize docs and was thinking to add the pr to this and found this. I think you should put a blurb in the docs linking to the official docs about public fields so that users can understand the need for the change and the model reference. This would be a great pr to merge.

[NGorco]

@blankstar85 Thanks, will do.

[NGorco]

@blankstar85 Added more info