Document credentials.json for testing

[Boehrsi]

Is your feature request related to a problem? Please describe.

As we now need the credentials.json for testing, we have to describe which structure and content must be given. As this file contains login information it's not available via this repository.

Describe the solution you'd like

• Add documentation to our testing wiki page https://github.com/open-xchange/ox-coi/wiki/Testing
• Document where the file is located
• Document the required JSON structure
• Show an example which data must be added to the JSON (without exposing real data of course)

Internal Bug Tracker.