Add section on user data and privacy

[adamsilverstein]

With the introduction of the GDPR (https://www.eugdpr.org/) and increasing concern of privacy and user data online, it would be good to update our Engineering Best Practices to include a section on user data and privacy handling. Practices for cookies, consent and data collection and storage. Practices can closely follow the GDPR rules as a starting place.

[adamsilverstein]

See also: https://make.wordpress.org/core/2018/03/28/roadmap-tools-for-gdpr-compliance/