Write a guide on environment setup, troubleshooting, and preparations (e.g. downloads) #71

We should collect these information in a central place (maybe a document in this repo) and send it out to participants before the event.

Also a FAQ covering things like

• I am seeing some unrelated test failures, is that OK?
• What does it mean to replace fixturesDir with fixtures?
• Which branch should I target?
• Reviewers asked me to rebase, what does it mean?
• etc.

would be very useful.