chore: add FIXME explaining why bevy0.4 isn't compatible

src/main.rs

@@ -93,6 +93,9 @@ fn mandelbrot_render_system(
                         iters,
                     );
 
+                    // FIXME: This will not work due to https://github.com/bevyengine/bevy/issues/1161
+                    // The workaround involves "getting a mutable borrow of the ColorMaterial you use to display the Texture"
+                    // according to @FrancoisM#2474, a developer of Bevy on Discord
                     texture.data.copy_from_slice(buf);
                 }
             }