document: config file moved from /usr/src to /usr/src/app

[billchurch]

via @israel-tsadok-silk from #368

In the docker image, the config file moved from /usr/src to /usr/src/app, and also now the default config "just works", so it just silently failed to take my config file, which made it tricky to figure out what happened. The new location makes more sense, but maybe this needs to be highlighted somewhere, or a warning should be displayed if a config.json file is found under /usr/src.