Added a note for password change

lab-vm/README.md

@@ -69,6 +69,13 @@ vm-with-packer
 
 - Take a look at the [wiki](https://code.fbi.h-da.de/danet/gosdn/-/wikis/Deployment/Virtual-Machine-with-Packer).
 
+### ⚠️ Note
+
+If you want to change the password of the user **gosdn** for Ubuntu Server a new one has to be created with the command **mkpasswd -m sha512crypt newpassword** (**newpassword** is a placeholder for the password you want to use). **mkpasswd** is in the **whois** package for Debian/Ubuntu.
+Alternatively you can use the command **openssl passwd -6 newpassword** if you can not use **mkpasswd**.
+
+You **can not** set the password in **plaintext** in meta-data!
+
 ---
 
 ## References