Add comment about where imports.css was generated

buildutils/src/build.ts

@@ -165,6 +165,8 @@ export namespace Build {
 
       // Template the CSS index file.
       let cssContents = '/* This is a generated file of CSS imports */';
+      cssContents +=
+        '\n/* It was generated by @jupyterlab/buildutils in Build.ensureAssets() */';
       cssContents += `\n@import url('~${appCSS}');`;
       cssImports.forEach(cssImport => {
         cssContents += `\n@import url('~${cssImport}');`;