Write instructions for db migrations

[bajiat]

From now on, all developers should handle migration in their PR, if their task includes changes to database: adding or removing collections, renaming collections, adding or removing fields.

In order for the developers to know how to do this, there should be instructions on writing migration functions/scripts.

Definition of done

• Come up with a proposal for instructions
• Check the proposal with team
• Finalize the instructions and create a PR