Document dollar notation (env var) values in app.config.yaml

[shazron]

See https://github.com/apache/openwhisk-wskdeploy/blob/master/specification/html/spec_parameters.md#dollar-notation--schema-for-values

There is no reference to this in our docs.

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/AIOE-670

[purplecabbage]

This should be done along with the refactor to do interpolation in 1 place only.
The dev command provides a robust implementation and supports

$DEFAULT_USERNAME
${DEFAULT_USERNAME}
{DEFAULT_USERNAME}

and will ignore literals

'$DEFAULT_USERNAME'
"$DEFAULT_USERNAME" 
...

[shazron]

there are already docs https://developer.adobe.com/app-builder/docs/guides/configuration/#using-environment-variables-in-runtime-actions

@purplecabbage maybe a feature request paired with the implementation PR