Note private env var handling with cloudflare builds (#4490)

packages/integrations/cloudflare/README.md

@@ -55,3 +55,17 @@ In order to work around this:
 - install the `"web-streams-polyfill"` package
 - add `import "web-streams-polyfill/es2018";` to the top of the front matter of every page which requires streams, such as server rendering a React component.
 
+## Environment Variables
+
+As Cloudflare Pages Functions [provides environment variables differently](https://developers.cloudflare.com/pages/platform/functions/#adding-environment-variables-locally), private environment variables needs to be set through [`vite.define`](https://vitejs.dev/config/shared-options.html#define) to work in builds.
+
+```js
+// astro.config.mjs
+export default {
+  vite: {
+    define: {
+      'process.env.MY_SECRET': JSON.stringify(process.env.MY_SECRET),
+    },
+  },
+}
+```