Update certificates.md (#3078)

docs/admin/vm-ops/certificates.md

@@ -5,5 +5,12 @@ tags:
   - operations
 ---
 
-This is a placeholder
+We are using LetsEncrypt cert to write this documentation. 
+
+ - Install repository: `apt-get install software-properties-common` 
+ - Update system: `apt-get update`
+ - Install certbot: `apt-get install python3-certbot-apache` 
+ - Issue certificate: `certbot --apache -d fqdn_of_Gluu_server`
+ - Cert and key are available in: `/etc/letsencrypt/live/` location
+ - Reboot your server