docs: explain tranformIgnorePatterns option usage

in cli-plugin-jest README

Author: LinusBorg

packages/@vue/cli-plugin-unit-jest/README.md

@@ -38,3 +38,32 @@ Jest can be configured via `jest.config.js` in your project root, or the `jest`
 ``` sh
 vue add @vue/unit-jest
 ```
+
+## Transform dependencies from `/node_modules`
+
+By default, jest doesn't transform anything from `/nodee_modules`.
+
+Since jest runs in node, we also don't have to transpile anything that uses modern ECMAScript features as Node >=8 already supports these features, so it's a sensible default. cli-plugin-jest also doesn't respect the `transpileDependencies` option in `vue.config.js` for the same reason.
+
+However, we have two cases where we do need to transpile code from `/node_modules` in jest:
+
+1. Usage of ES6 `import`/`export` statements, which have to be compiled to commonjs `module.exports`
+2. Single File Components (`.vue` files) which have to be run through `vue-jest`
+
+To do this, we need to add an exception to the `tranformIgnorePatterns` option of jest. This is its default value:
+
+```js
+tranformIgnorePatterns: ["/node_modules/"]
+```
+
+We have to add exceptions to this pattern with a RegExp negative lookahead:
+
+```js
+tranformIgnorePatterns: ["/node_modules/(?!name-of-lib-o-transform)"]
+```
+
+To exclude multiple libraries:
+
+```js
+tranformIgnorePatterns: ["/node_modules/(?!lib-to-transform|other-lib)"]
+```