Add stefans comments from mattermost to signup page

Fixes:

Stefan Hugtenburg
11:36 AM

Zie net dat de tekst nog niet helemaal klopt 
Het is de Engelse test voor TA'ing (checkbox 1) en de TA-training (checkbox 2) (edited)

merged

By Ruben Keulemans on 2018-05-23T09:44:42 (imported from GitLab project)

mentioned in commit 5d244715

By Ruben Keulemans on 2018-05-23T09:44:42 (imported from GitLab project)