Update server-islands.mdx to add caching section

src/content/docs/en/guides/server-islands.mdx

@@ -71,6 +71,14 @@ Each island is loaded independently from the rest. This means a slower island wo
 
 This rendering pattern was built to be portable. It does not depend on any server infrastructure so it will work with any host you have, from a Node.js server in a Docker container to the serverless provider of your choice.
 
+## Caching
+
+The data for server islands is retrieved via a `GET` request, passing props as an encrypted string in the URL query. This allows caching data with the [`Cache-Control` HTTP header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control) using standard `Cache-Control` directives.
+
+However, [the browser limits URLs to a maximum length of 2048 bytes](https://chromium.googlesource.com/chromium/src/+/master/docs/security/url_display_guidelines/url_display_guidelines.md#url-length) for practical reasons and to avoid causing denial-of-service problems. If your query string causes your URL to exceed this limit, Astro will instead send a `POST` request that contains all props in the body. 
+
+`POST` requests are not cached by browsers because they are used to submit data, and could cause data integrity or security issues. Therefore, any existing caching logic in your project will break. Whenever possible, pass only necessary props to your server islands and avoid sending entire data objects and arrays to keep your query small.
+
 ## Accessing the page URL in a server island
 
 In most cases you, your server island component can get information about the page rendering it by [passing props](/en/basics/astro-components/#component-props) like in normal components.