Document EXACT behavior of override_with_env_vars

[cbrunnkvist]

As far as I can understand by re-reading the README, the behavior should be like this:

Default is value substitution only:

# If environment variables "value1_from_env=yep" and "key1=nope" are defined
# Then `key1` will have value "yep"
key1 = ${value1_from_env}

then we get to the interesting part,

With -Dconfig.override_with_env_vars=true:

# If environment variable "CONFIG_FORCE_val1" is set, its value will be assigned to `key1`
key1 = "val1"

fine,

However a quirk that seems to be missing from the current docs is that with -Dconfig.override_with_env_vars=true:

# For top-level / non-mangled keys only
# If environment variable "key1=val1" is set, its value will be assigned to `key1` DESPITE not being prefixed with "CONFIG_FORCE_"
# But 
# If a "mangled" environment variable "CONFIG_FORCE_key1=val2" is also set, ITS value will take precedence
key1 = "val1"

[cbrunnkvist]

If this is a bug, I suspect that it will be difficult to fix without a major version bump, because quite a few projects are likely to already depend on this behavior.

(N.b. I tested through Scala only)