doc: Add warning in index.html

src/index.html

@@ -1 +1,14 @@
+<!-- WARNING:
+
+    This file only exists for the build system to work properly.
+    Any changes done here won't be reflected in the final build.
+
+    The actual loading of `renderer.ts` is done in `src/index.ts`
+    within the `createMainWindow` function.
+
+    Archived link for reference:
+    https://github.com/th-ch/youtube-music/blob/a3104fda4b0d58b076d0c737111636a66e468acc/src/index.ts#L407-L443
+
+-->
+
 <script type="module" src="./renderer.ts"></script>