feat: bring in Sandia ECS

Author: timyardley

tools/testbeds/README.md

@@ -26,6 +26,7 @@ Developed as a community asset
 - [SWaT Simulator](https://github.com/yuqiChen94/Swat_Simulator) - SUTD SWaT Testbed Simulation written in python
 - [Simple Process Simulation](https://github.com/arnaudsoullie/simple-process-simulation) - HTML/JS process simulation interacting with a modbus device, by Arnaud Soullie
 - [ICSSIM](https://github.com/AlirezaDehlaghi/ICSSIM) - A Framework for Building Industrial Control Systems Security Simulation Testbeds, [Paper](https://arxiv.org/abs/2210.13325)
+- [Sandia Experiment Control System](https://github.com/sandialabs/ECS) - The purpose of the Sandia Experimental Control System (ECS) is to provide a holistic cyber-physical environment for experiments on the response of organizations and their personnel to cyber-physical blended attacks.