vm: add warning about the CRI not supporting this setup

Signed-off-by: Marin Hannache <mareo@cri.epita.fr>

docs/cri_vm.md

 # How to setup and use the PIE VM
 
+!!! warning
+    No support is done by the CRI on the PIE VM. If you do not succeed in
+    making it work by your own means, you must work on the school computers.
+
 This method allow you to have the PIE environment running in a VM on your
 computer.