Update DOCS.md on location of apps source files

[gmeisl]

Problem/Motivation

It seems that with the AppDaemon Addon the source code for apps should be placed in /root/addon_configs/a0d7b954_appdaemon/apps and not under config/apps (as written in all AppDaemon documentation/tutorials that I found)

Expected behavior

DOCS.md should state where application code should be located (and also other potential differences to https://appdaemon.readthedocs.io/en/latest/index.html )

Proposed changes

More explicit documentation. Thanks!