Add example to student number field

Ghost User
2019-10-02T19:11:45+02:00

Closes #908

#### Previous behaviour
Steps to reproduce:

1. Register as member
2. Unclear what the value for student number should be

#### New behaviour
Steps to validate that it works:

1. Register as member
2. The example shows you exactly what to do

![Screenshot_2019-09-08_at_11.03.06](/uploads/06e504061222f65f1a32c89e2c73fc90/Screenshot_2019-09-08_at_11.03.06.png)
(this screenshot is missing the full stop at the end but I'm too lazy to make a new one)