doc: add documentation for assetExtensions configuration option

README.md

@@ -112,6 +112,14 @@ module.exports = createConfig({
    */
   extensions: ['.js', '.ts', '.jsx', '.tsx'],
 
+  /**
+   * define list of file extensions to be considered as asset files.
+   * Asset files are copied over to the build directories.
+   *
+   * example, .json, .png, .jpg, .jpeg, etc
+   */
+  assetExtensions: []
+
   /**
    * defines string of file patterns to process
    */