Document configuration precedence and behaviour

[drewsonne]

Expected Behavior

There should be more details regarding the explicit behaviour of the precedence between config file, CLI options, and environment variables.

Current Behavior

There is a description of what the config options do, but not which sources have precedence.

Possible Solution

Write a section in the readme describing this.

Context (Environment)

When trying to deploy this en masse, I noticed that the seconds_duration option was being overridden.
During development, there was inconsistency in the approach taken to handling configuration and some documentation would crystalise this meaning