Add notice on pre-built releases (#57)

Release version notice in Readme.

Author: chenjunnn

README.md

@@ -34,6 +34,8 @@ This asset can be prepared in two flavours:
 
 ## Releases
 
+> **Note:** The releases are built in overlay mode.
+
 The best way to start quickly is to use our releases.
 
 You can download pre-built [releases](https://github.com/RobotecAI/ros2-for-unity/releases) of the Asset that support both platforms and specific ros2 and Unity3D versions.