Update securing-your-github-pages-site-with-https.md (#26373)

Co-authored-by: Sophie <29382425+sophietheking@users.noreply.github.com>

Author: evK1

content/pages/getting-started-with-github-pages/securing-your-github-pages-site-with-https.md

@@ -40,6 +40,8 @@ All {% data variables.product.prodname_pages %} sites, including sites that are
 
 When you set or change your custom domain in the Pages settings, an automatic DNS check begins. This check determines if your DNS settings are configured to allow {% data variables.product.prodname_dotcom %} to obtain a certificate automatically. If the check is successful, {% data variables.product.prodname_dotcom %} queues a job to request a TLS certificate from [Let's Encrypt](https://letsencrypt.org/). On receiving a valid certificate, {% data variables.product.prodname_dotcom %} automatically uploads it to the servers that handle TLS termination for Pages. When this process completes successfully, a check mark is displayed beside your custom domain name.
 
+Please note that your {% data variables.product.prodname_pages %} site must be publicly available for a Let's Encrypt certificate to be issued. Once the certificate has been issued you may revert the site to private. 
+
 The process may take some time. If the process has not completed several minutes after you clicked **Save**, try clicking **Remove** next to your custom domain name. Retype the domain name and click **Save** again. This will cancel and restart the provisioning process.
 
 ## Resolving problems with mixed content