docs: add matrix math prerequisite note to What's Next

docs/01-rainbow-triangle.md

@@ -1371,4 +1371,19 @@ With the render loop and pipeline foundation in place, the next steps are:
 - **Compute shaders and GPU compute
   pipelines** — general-purpose GPU computation outside the graphics pipeline
 
+> **Prerequisite note — matrix math:** Every topic above ultimately depends on
+> matrix mathematics. Transforms (model, view, and projection matrices) move
+> geometry from local object space through world space, camera space, and
+> finally into clip space. In this tutorial, all vertex positions are hardcoded
+> NDC coordinates so we can focus on the rendering pipeline itself. Real
+> applications compute these coordinates via matrix multiplication: a
+> transformation matrix is uploaded to the GPU as a uniform, and the vertex
+> shader multiplies each vertex by that matrix before outputting
+> `clip_position`. If linear algebra is unfamiliar, study it before diving
+> into the next tutorials. Recommended resources: [Learn
+> OpenGL's linear algebra section](https://learnopengl.com/Getting-started/Coordinate-Systems)
+> for a graphics-oriented treatment, and
+> [3Blue1Brown's Essence of Linear Algebra](https://www.youtube.com/playlist?list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab)
+> for an intuitive visual foundation.
+
 Keep [concepts/GLOSSARY.md](concepts/GLOSSARY.md) handy as you move forward.