add guide

include/Engine.hpp

@@ -6,7 +6,7 @@
 
 struct GLFWwindow;
 
-namespace planetarium {
+namespace engine {
 
 // -----------------------------------------------------------------------
 // Engine
@@ -15,9 +15,9 @@ namespace planetarium {
 // routing between raw GLFW callbacks and the Camera. This is the only
 // class that knows about GLFW directly.
 //
-// This is the reusable "engine" half of the original Planetarium project
-// with every game-specific piece (SolarSystem, Planet, UI, picking)
-// removed. Hook your own logic in via setUpdateCallback/setRenderCallback:
+// A generic reusable "engine" shell: window/context setup, the main loop,
+// and camera input routing, with no game-specific logic baked in. Hook
+// your own logic in via setUpdateCallback/setRenderCallback:
 //
 //     Engine engine;
 //     engine.initialize(1600, 900, "My App");
@@ -74,4 +74,4 @@ private:
     double lastFrameTime_ = 0.0;
 };
 
-}  // namespace planetarium
+}  // namespace engine