Clarify why we only need css?-autoprefixer in development

Fixes #377

config/webpack.config.prod.js

@@ -116,7 +116,12 @@ module.exports = {
         include: [paths.appSrc, paths.appNodeModules],
         // "?-autoprefixer" disables autoprefixer in css-loader itself:
         // https://github.com/webpack/css-loader/issues/281
-        // We already have it thanks to postcss.
+        // We already have it thanks to postcss. We only pass this flag in
+        // production because "css" loader only enables autoprefixer-powered
+        // removal of unnecessary prefixes when Uglify plugin is enabled.
+        // Webpack 1.x uses Uglify plugin as a signal to minify *all* the assets
+        // including CSS. This is confusing and will be removed in Webpack 2:
+        // https://github.com/webpack/webpack/issues/283
         loader: ExtractTextPlugin.extract('style', 'css?-autoprefixer!postcss')
         // Note: this won't work without `new ExtractTextPlugin()` in `plugins`.
       },