Hi.

I couldn't find any documentation reference for including JVM properties and good old environment variables as configuration source. Due to the popularization of Docker philosophy and 12-factor application concept it would be quite nice to provide library users with simple way to use both sources in their apps. I've already found a workaround with InMemoryConfigurationSource (and i know that documentation will become confusing the very moment it will specify word 'environment' in two meanings), however, i feel that is necessary to update library in that way (or at least mention InMemoryConfigurationSource workaround in documentation).