Add let's encrypt omnibus guide link

doc/administration/container_registry.md

@@ -175,6 +175,10 @@ certificate for that specific domain (e.g., `registry.example.com`) or maybe
 a wildcard certificate if hosted under a subdomain of your existing GitLab
 domain (e.g., `registry.gitlab.example.com`).
 
+NOTE: **Note:**
+As well as manually generated SSL certificates (explained here), certificates automatically
+generated by Let's Encrypt are also [supported in Omnibus installs](https://docs.gitlab.com/omnibus/settings/ssl.html#host-services).
+
 Let's assume that you want the container Registry to be accessible at
 `https://registry.gitlab.example.com`.