Add missing comma to default config in README

Author: robhowell

README.md

@@ -80,7 +80,7 @@ If you pass one of the top-level options in (`extensions`, `transform`, etc), it
         ast: false,
         babelrc: false,
         plugins: [
-          'babel-plugin-add-module-exports'
+          'babel-plugin-add-module-exports',
           '@babel/plugin-proposal-class-properties',
           '@babel/plugin-proposal-object-rest-spread',
           '@babel/plugin-transform-runtime',