Document serialization of data (vercel#3516)

* Document serialization of data

* Add date

readme.md

@@ -249,6 +249,8 @@ export default class extends React.Component {
 
 Notice that to load data when the page loads, we use `getInitialProps` which is an [`async`](https://zeit.co/blog/async-and-await) static method. It can asynchronously fetch anything that resolves to a JavaScript plain `Object`, which populates `props`.
 
+Data returned from `getInitialProps` is serialized when server rendering, similar to a `JSON.stringify`. Make sure the returned object from `getInitialProps` is a plain `Object` and not using `Date`, `Map` or `Set`.
+
 For the initial page load, `getInitialProps` will execute on the server only. `getInitialProps` will only be executed on the client when navigating to a different route via the `Link` component or using the routing APIs.
 
 _Note: `getInitialProps` can **not** be used in children components. Only in `pages`._