Add instructions for UEFI, even if image is broken

I know, not the cleanest way as it is a broken image that we just use to manually request PXEBoot, but still a way to test UEFI changes now that we do not have access to physical hardware.

In any case, the description has been tested both for aimstest and aims and installation begins (UEFI VM using aims prod):

README.md

 # aims2-ipxe
 
+## BIOS approach
+
 As of August 2019, OpenStack cells running Neutron at CERN do not currently support DHCP passthrough of the 'filename' and 'next-server' variables.
 
-This repository exists to ensure it's always possible to boot a virtual machine that will work with AIMS2, regardless of the underlying network infrastructure.
+This repository exists to ensure it's always possible to boot a BIOS virtual machine that will work with AIMS2, regardless of the underlying network infrastructure.
 
 The script `generate-ipxe-image.sh` will create an iPXE image file that contains the variables 'filename' and 'next-server' hardcoded inside the configuration.
 
@@ -46,3 +48,17 @@ Once the image is generated, you can upload the image to OpenStack glance from a
 $ `eval $(ai-rc "IT Linux Support - CI")`
 
 $ `openstack image create --private --disk-format raw --file iPXE-20190801-aimsprod.img iPXE-20190801-aimsprod`
+
+## UEFI approach
+
+For UEFI, the alternative is to generate the same image but to upload it according to the [Cloud UEFI docs](https://clouddocs.web.cern.ch/advanced_topics/uefi_support.html#how-to-upload-an-uefi-compatible-image-to-the-cern-cloud). This image **will not boot** as of today, but we must specify an image for a VM.
+
+**This means that it required to set the LanDB::Os to 'LINUX' (aims) or 'PXELINTEST' (aimstest) when booting an OpenStack VM when using this image.**
+
+The generated image must be uploaded with specific properties:
+
+$ `eval $(ai-rc "IT Linux Support - CI")`
+
+$ `openstack image create --private --disk-format raw --file iPXE-20190801-aimsprod.img iPXE-20190801-aimsprod-uefi --property hw_firmware_type='uefi' --property hw_machine_type='q35' --property architecture='x86_64'`
+
+The uploaded image will not boot, press ENTER to get into the boot manager menu. Select `Boot Manager` -> `UEFI PXEv4`. This will start the default AIMS2 menu for UEFI entries.