Added materials tab

materials/python-w-unreal-engine/PLACEHOLDER.txt

@@ -0,0 +1,8 @@
+Put the real files here, named exactly:
+
+  slides.pdf   — the presentation
+  project.zip  — the Unreal Engine project / code
+
+materials.html links directly to these two filenames, so once you drop
+them in this folder the download links on the site start working —
+no HTML changes needed. Delete this placeholder file afterwards.