New Practice: Distinguish STDOUT from STDERR

[jasonkarns]

I was about to open a PR that adds notes specific to distinguishing STDOUT from STDERR. However, the concept would have been dispersed among many other items:

• Enable structured output
• Informational errors
• Provide debug mode

As the topic of stdout vs stderr is big enough, I think it probably warrants its own line-item.

Key point: STDERR is for more than just error output. It's for all diagnostic output.