chore(babel-preset): create readme with basic docs (#5127)

packages/babel-preset-app/README.md

@@ -0,0 +1,64 @@
+# `@nuxt/babel-preset-app`
+> Default babel preset for nuxt
+
+## Usage
+
+This is the default preset used by Nuxt, which is mainly a wrapper around the `@babel/preset-env` preset. It also optionally uses the `@vue/babel-preset-jsx` preset as well as `@babel/plugin-syntax-dynamic-import`, `@babel/plugin-proposal-decorators`, `@babel/plugin-proposal-class-properties`, `@babel/plugin-transform-runtime`. Furthermore the preset is adding polyfills.
+
+Usually, no additional configuration is required. If needed though, there is an option to fine-tune the preset's behavior. Just add the following to `nuxt.config.js`:
+```js
+babel: {
+  presets({ isServer }) {
+    return [
+      [ "@nuxt/babel-preset-app", options ]
+    ]
+  }
+}
+```    
+...where `options` is an object with parameters, for example:
+```
+ const options = {
+  useBuiltIns: "entry"
+}
+```
+Below is a list of all available parameters:
+
+### Options
+* **buildTarget** - passed in through the Builder, either `"server"` or `"client"`
+* **configPath** - [`@babel/preset-env` parameter](https://babeljs.io/docs/en/babel-preset-env#configpath)
+* **forceAllTransforms** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#forcealltransforms)' parameter
+* **debug**, default  `false` - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#debug)' parameter
+* **decoratorsBeforeExport**
+* **decoratorsLegacy**, default true
+* **exclude** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#exclude)' parameter
+* **ignoreBrowserslistConfig**, defaults to value of `modern` - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#ignorebrowserslistconfig)' parameter
+* **include** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#include)' parameter
+* **jsx**, default truish, can be a an object passed as params to [@vue/babel-preset-jsx`](https://www.npmjs.com/package/@vue/babel-preset-jsx)
+* **loose**, default `false` - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#loose)' parameter and also sets `loose=true` for `@babel/plugin-proposal-class-properties`
+* **modern** passed by builder, either `true` or `false`
+* **modules**, default `false` - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#modules)' parameter
+* **polyfills**, default `['es6.array.iterator','es6.promise','es7.promise.finally']`, more [in the corresponding repository](https://github.com/zloirock/core-js)
+* **shippedProposals** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#shippedproposals)' parameter
+* **spec** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#spec)' parameter
+* **targets** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#targets)' parameter
+* **useBuiltIns**, default `"usage"` - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#usebuiltins)' parameter
+
+There are [detailed docs](https://babeljs.io/docs/en/babel-preset-env#options) for the parameters of '@babel/preset-env'.
+
+### Example 1. Change targets for server and client respectively
+```js
+babel: {
+  presets({ isServer }) {
+    return [
+      [
+        "@nuxt/babel-preset-app",
+        {
+          targets: isServer
+            ? { node: "current" }
+            : { browsers: ["last 2 versions"], ie: 11 }
+        }
+      ]
+    ]
+  }
+},
+```