[skip ci] docs: add version notice for main process env vars

Author: nklayman

docs/guide/guide.md

@@ -97,7 +97,7 @@ console.log(fileContents)
 
 ## Env Variables
 
-Read [Vue ClI's documentation](https://cli.vuejs.org/guide/mode-and-env.html) to learn about using environment variables in your app. All env variables prefixed with `VUE_APP_` will be available in both the main and renderer processes.
+Read [Vue ClI's documentation](https://cli.vuejs.org/guide/mode-and-env.html) to learn about using environment variables in your app. All env variables prefixed with `VUE_APP_` will be available in both the main and renderer processes (Only available in main process since `1.0.0-rc.4`).
 
 ## How it works