add warning about jest

to transpileDependencies docs

Author: LinusBorg

docs/config/README.md

@@ -187,6 +187,14 @@ Deprecated since Vue CLI 3.3, please use [`publicPath`](#publicPath) instead.
 - Default: `[]`
 
   By default `babel-loader` ignores all files inside `node_modules`. If you want to explicitly transpile a dependency with Babel, you can list it in this option.
+  
+::: warning Jest config
+This option is not resprected by the [cli-unit-jest plugin](#jest), because in jest, we don't have to transpile stuff from `/node_modules` unless it uses non-standard features (node 8> supports the latest ECMAScript features already).
+
+However, jest sometimes has to transpile content from node_modules if the code that's consumed uses ES6 `import`/`export` syntax. In that case, use the `tranformIgnorePatterns` option in `jest.config.js`. 
+
+See [the plugin's README](https://github.com/vuejs/vue-cli/blob/dev/packages/%40vue/cli-plugin-unit-jest/README.md) for more information.
+:::
 
 ### productionSourceMap