Describe usecase: Injecting faults

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Add content to ./doc/usecases/injecting_faults.adoc.

• [ ] Add a description of the use case. Why would you want to do this? What options do you have? What are the most important considerations?
• [ ] Add prerequistes of this use case. What needs to be done before you can start this use case.
• [ ] Add relevant messages including a short description of how they are relevant to this use case.
• [ ] Add one or two examples.