Formalize a Design Guideline

[krestenlaust]

To make it easier for contributors to design features without as much back and forth, it would be neat to have a design guideline.

The guideline should describe

• The charming personality of the stregsystem
• The context of the system (e.g. trust, cashier-less, volunteer work) (to address Added a confirmation dialog to prevent accidental purchase #548, Replace id with username in url #286 and Can access other peoples login #345 )

What could also be covered

• Best practices for developing features (e.g. be aware of what app the feature should reside in, removing excess migrations in PRs)
• How products are expected to be populated (and used, e.g. headers for events)
• Workflows for volunteers
• The current deployment of the application on the server