docs(datatype): add a note about BIGINT datatype conversion to string (#5796)

Denis C de Azevedo
2016-04-26T21:48:01+02:00

* docs(datatype): add a note about BIGINT datatype conversion to string

This helps users to understand the reason to this conversion and avoid more open issues to Sequelize like these:
- https://github.com/sequelize/sequelize/issues/1774
- https://github.com/sequelize/sequelize/issues/4550

* docs(datatype): add documentation to source code