docs: Fallbacks from other locales (#124)

Author: amannn

packages/website/pages/docs/usage/error-handling.mdx

@@ -31,3 +31,20 @@ function getMessageFallback({namespace, key, error}) {
   <App />
 </NextIntlProvider>
 ```
+
+## Fallbacks from other locales
+
+If you have incomplete messages for a given locale and would like to use messages from another locale as a fallback, you can merge the two before passing them to `NextIntlProvider`.
+
+```jsx
+import deepmerge from 'deepmerge';
+
+// pages/index.js
+export async function getStaticProps({locale}) {
+  const userMessages = (await import(`../../messages/${locale}.json`)).default;
+  const defaultMessages = (await import(`../../messages/en.json`)).default;
+  const messages = deepmerge(defaultMessages, userMessages);
+
+  return {props: {messages}};
+}
+```