README: note another workaround

README.md

@@ -52,6 +52,9 @@ nix run .#server2-vm
 Login as `root` with an empty password. Use `build-container $(ls -1
 /etc/lxc/containers)` to build and start the required containers.
 
+If you encounter `too many open files` errors, bump limits for virtio
+with `ulimit -n 524288` before starting QEMU.
+
 #### Gerätekonfigurationen
 
 ```