Add "typical" mistakes to documentation

[racke]

E.g. for configuration:

• Duplicate keys
• Typo in keys
• Change in config.yml overridden by environments

[xsawyerx]

Where do you suggest putting this?

[cromedome]

We could either add a troubleshooting section to the cookbook perhaps, or a separate document altogether? My preference would be the cookbook. "What to do when things go wrong" or somesuch.