[docs] update sops

doc/src/anleitung/sops.md

@@ -23,3 +23,13 @@
 - Write `- *admin_$USER` under the apropriate `key_grups:` of the secrets the user should have access to
 
 - `cd machines/` and reencrypt existing secrets for the new key with `sops updatekeys $path/to/secrets.yaml`
+
+## How to add host keys
+If a new host is created we have to add its age keys to the sops config.
+Do the following:
+```bash
+# ssh into the host and run:
+nix-shell -p ssh-to-age --run 'cat /etc/ssh/ssh_host_ed25519_key.pub | ssh-to-age'
+# create new host with the output of that command in /machines/.sops.yaml
+```
+