feat(gdpr): add GDPR.txt file according to https://gdpr-txt.org/

The GDPR.txt file is a proposed standard which informs hosting providers about the personal data collected by softwares. It aims to simplify the compliance to the General Data Protection Regulation (GDPR) of hosting providers. Note that a GDPR.txt file is not enough to make your project GDPR compliant (but it will help).