update README.md to report 3.7 as the latest stable version (#2322)

Author: stephengold

README.md

@@ -4,7 +4,7 @@ jMonkeyEngine
 [![Build Status](https://github.com/jMonkeyEngine/jmonkeyengine/workflows/Build%20jMonkeyEngine/badge.svg)](https://github.com/jMonkeyEngine/jmonkeyengine/actions)
 
 jMonkeyEngine is a 3-D game engine for adventurous Java developers. It’s open-source, cross-platform, and cutting-edge.
-v3.6.1 is the latest stable version of the engine.
+v3.7.0 is the latest stable version of the engine.
 
 The engine is used by several commercial game studios and computer-science courses. Here's a taste: