add the Repository purpose on the top of the readme

Author: chamhoo

README.md

@@ -1,5 +1,15 @@
 ![Isaac Lab](docs/source/_static/isaaclab.jpg)
 
+---
+
+# Repository purpose
+
+This repository is a maintained snapshot of Isaac Lab tailored for our research and development:
+- The upstream Isaac Lab is actively developed; large changes between releases can introduce regressions and performance variations.
+- We will develop and maintain custom RL models and experiments on top of this codebase; keeping our own stable release ensures reproducibility and easier debugging.
+- We will periodically merge, test, and adopt upstream improvements to stay compatible with new features.
+
+
 ---
 
 # Isaac Lab