add SSL howto link

Author: coding-horror

docs/INSTALL-digital-ocean.md

@@ -172,6 +172,8 @@ Do you want...
 - Users to post reples via email? [Configure reply via email](https://meta.discourse.org/t/set-up-reply-via-email-support/14003).
 
 - Automatic daily backups? [Configure backups](https://meta.discourse.org/t/hot-off-the-presses-automated-backup-support/13805)
+ 
+- HTTPS / SSL support? [Configure SSL](https://meta.discourse.org/t/allowing-ssl-for-your-discourse-docker-setup/13847)
 
 If anything needs to be improved in this guide, feel free to ask on [meta.discourse.org][meta], or even better, submit a pull request.