Thalia Pay: Add user-instructions for activating in bankaccounts

One-sentence description

Add information about Thalia Pay to bankaccounts

Motivation

People should know what Thalia Pay is and how to activate it.

Desired functionality

Something about that if you sign a direct debit, it will be used for Thalia Pay etc.

Suggested implementation

Maybe ask the board? Or just come up with something ourselves. It should not be too extensive in my opinion, just something short.