Add an extra note to explain the typeof window !== "undefined" checks.

Author: alexanderson1993

src/pages/useLocalStorage.md

@@ -15,6 +15,9 @@ Sync state to local storage so that it persists through a page refresh.
 Usage is similar to useState except we pass in a local storage key so that we can
 default to that value on page load instead of the specified initial value.
 
+Since the local storage API isn't available in server-rendering environments, 
+we check that `typeof window !== "undefined"` to make SSR and SSG work properly.
+
 ```jsx
 import { useState } from "react";