Document how to build for older browsers

Document how to transpile your node_modules with babel to be able to target older browsers.

docs/getting-started/installation.md

@@ -14,3 +14,29 @@ yarn add react-async
 
 > This package requires `react` as a peer dependency. Please make sure to install that as well. If you want to use the
 > `useAsync` hook, you'll need `react@16.8.0` or later.
+
+## Targeting older browsers
+
+If you are targeting older browsers you need to transpile `react-async` with babel.
+
+To transpile `node_modules` with babel you need to use a `babel.config.js`, for more information see [babel's documentation](https://babeljs.io/docs/en/configuration#whats-your-use-case).
+
+In your `webpack.config.js` make sure that the rule for `babel-loader`:
+  * doesn't exclude `node_modules` from matching via the `exclude` pattern;
+  * excludes `core-js` as it shouldn't be transpiled;
+  * is passed the `configFile` option pointing to the `babel.config.js` file.
+
+```
+{
+  test: /\.(js|jsx)$/,
+  exclude: /\/node_modules\/core-js\//,
+  use: [{
+    loader: 'babel-loader',
+    options: {
+      configFile: './babel.config.js',
+      // This is recommended to enable when transpiling node_modules to improve build times for consecutive builds.
+      cacheDirectory: true,
+    }
+  }]
+}
+```