Add qemu support for Lagarto Ox

The network is still not working automatically, but the eth0 device is
present in the host.

Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

README.md

@@ -45,6 +45,20 @@ $ picocom -q -b 115200 $FPGACTL_UART
 
 It should boot without any user interaction.
 
+## Lagarto Ox on QEMU
+
+To build the system and boot it on QEMU, you can run the following:
+
+```
+$ nix develop -L '.#qemu-lagarto-ox'
+```
+
+Then, simply run boot.sh to start the QEMU system.
+
+```
+$ ./boot.sh
+```
+
 ## Lagarto Hun
 
 WIP