Remove outdated note in README.md (#73)

As current releases support both overlay and standalone mode.

Author: chenjunnn

README.md

@@ -37,8 +37,6 @@ 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.