note polyfill requirement

Author: existentialism

packages/babel-preset-react-app/README.md

@@ -23,3 +23,5 @@ Then create a file named `.babelrc` with following contents in the root folder o
     "presets": ["react-app"]
   }
   ```
+
+This preset uses the `useBuiltIns` option with [transform-object-rest-spread](http://babeljs.io/docs/plugins/transform-object-rest-spread/), which assumes `Object.assign` is available or polyfilled.

packages/babel-preset-react-app/index.js

@@ -15,7 +15,8 @@ const plugins = [
     require.resolve('babel-plugin-transform-class-properties'),
     // { ...todo, completed: true }
     [require.resolve('babel-plugin-transform-object-rest-spread'), {
-      // Use Object.assign directly, instead of extends helper
+      // Use Object.assign directly, instead of extends helper. Note that this
+      // assumes Object.assign is available/polyfilled.
       useBuiltIns: true
     }],
     // function* () { yield 42; yield 43; }