doc: add vm external

docs/vm_external.md

+# How to setup and use pie-external VM
+
+This method allow you to have the PIE's environment in a VM on your computer.
+
+## First step
+
+Install [Virtual Box](https://www.virtualbox.org/).
+
+On linux you can install it with your packet manager (for [Debian](https://wiki.debian.org/VirtualBox) or [ArchLinux](https://wiki.archlinux.org/index.php/VirtualBox)).
+
+## Second step
+
+Download the VirtualBox appliance from [this address](https://static.cri.epita.fr/cri-vm.ova).
+
+Open VirtualBox and go to `File > Import Appliance` and import the file you just downloaded.
+
+![appliance](assets/vm_external_1.PNG)
+
+When setting `Appliance settings` verify RAM parameters as minimum is `2048 MB`.
+
+You can then click the `Import` button.
+
+![import](assets/vm_external_2.PNG)
+
+![folder](assets/vm_external_3.PNG)
+
+![settings](assets/vm_external_4.PNG)
+
+## Third step
+
+Inside Virtual Box select your newly created virtual machine and click on `Start` button.
+
+In the PXE menu, you should be used to see it, select `Prepare disk`.
+
+Let it boot, a script will run automatically. Your VM will shutdown after script execution.
+
+![select](assets/vm_external_5.PNG)
+
+![menu](assets/vm_external_6.PNG)
+
+## Fourth step
+
+You can now `Start` your VM again. You can now use the PXE menu as usual to boot `Archlinux` image.
+
+Be careful ! You should log with `epita` user and no password. Your data is in `/home/epita`, this directory is persistent so you will not lose files when rebooting.