docs: add note for sync issue with private github repo

resolves #408

docs/content/1.docs/1.getting-started/3.deploy.md

@@ -181,9 +181,13 @@ The GitHub Action provides the following outputs that you can use in subsequent
 
 NuxtHub automatically copies all your project's environment variables to your GitHub repository actions environment variables.
 
-When encrypting an environment variable in the NuxtHub Admin, a GitHub actions secret will be created in your repository
+When encrypting an environment variable in the NuxtHub Admin, a GitHub actions secret will be created in your repository.
 
-In order to use the GitHub Actions secret, you need to update the GitHub Actions workflow to use the secret as environment variable:
+::warning
+If you have a private repository on a free GitHub account or organization, NuxtHub won't be able to sync the env variables & secrets as GitHub repository environments (production / preview) are not available.
+::
+
+In order to use GitHub Actions secret, you need to update the GitHub Actions workflow to use the secret as environment variable:
 
 ```diff [.github/workflows/nuxthub.yml]
   - name: Build application