add guide

include/GLFunctions.hpp

@@ -13,7 +13,7 @@
 // loaders are mixed in the same program - see the comment at the top of
 // that file for details.
 //
-// Usage: call planetarium::gl::loadGLFunctions() exactly once, right
+// Usage: call engine::gl::loadGLFunctions() exactly once, right
 // after the GLFW OpenGL context has been made current and before any
 // other GL call in the project's own rendering code.
 // -----------------------------------------------------------------------
@@ -68,7 +68,7 @@ typedef char GLchar;
 typedef ptrdiff_t GLsizeiptr;
 typedef ptrdiff_t GLintptr;
 
-namespace planetarium::gl {
+namespace engine::gl {
 
 using PFNGLGENVERTEXARRAYS = void (*)(GLsizei, GLuint*);
 using PFNGLBINDVERTEXARRAY = void (*)(GLuint);
@@ -143,4 +143,4 @@ extern PFNGLGENERATEMIPMAP glGenerateMipmap;
 // required entry point could not be resolved.
 bool loadGLFunctions();
 
-}  // namespace planetarium::gl
+}  // namespace engine::gl