README: Add environment variable to Cloudflare Pages build

Author: winwiz1

README.md

@@ -350,11 +350,12 @@ If your SPA configuration block is different, then the newly built and deployed
     | Build command  | `yarn build:jamstack` |
     | Build output directory  | `client/dist`  |
 
-    Add the following environment variable:
+    Add the following environment variables:
 
     | Environment variable | Value |
     | :-------------------- | :----- |
     | NODE_VERSION | `16.13.0` |
+    |NODE_ENV|`production`|
 
     Optionally, you can customize the "Project name" field. It defaults to the GitHub repository name, but it does not need to match. The "Project name" is used to create a unique `*.pages.dev` subdomain. If the name is unique, it will be used as is, otherwise it will be altered a bit to ensure uniqueness. The resulting subdomain will be referred to as 'per-project subdomain' e.g. `<per-project>.pages.dev`.