Update README

README.md

@@ -94,6 +94,8 @@ This playbook sets up your server using the following Docker images:
 
 - [nginx](https://hub.docker.com/_/nginx/) - the [nginx](http://nginx.org/) web server (optional)
 
+- [certbot/certbot](https://hub.docker.com/r/certbot/certbot/) - the [certbot](https://certbot.eff.org/) tool for obtaining SSL certificates from [Let's Encrypt](https://letsencrypt.org/)
+
 
 ## Deficiencies