small readme change

Author: yamilbknsu

README.md

@@ -3,6 +3,8 @@
 [![Docs](https://github.com/NREL/DEMOS_NREL/actions/workflows/docs.yml/badge.svg)](https://nrel.github.io/DEMOS_NREL/)
 
 ## Usage
+> A public Docker image of DEMOS has not been released. Please follow the `From Source` instructions.
+
 ### Docker Container
 The docker image for demos is stored in `registry/demos:latest`. The input data and configuration file are fed to the container through volumes. Alternatively, we provide a `docker-compose` workflow that can be used.