Add simple build/run tutorial for MIT's xv6 OS

[ivandavidov]

xv6 is a very small UNIX-like operating system which can be built from scratch. There is also a teaching book which explains the internals of the xv6 OS.

xv6 can be mentioned in the MLL docs as additional educational resource for the people who are interested to build their own UNIX-like OS. This could be mentioned in the website, the MLL tutorial document and in the project's "readme" document, since the build/run steps are really simple.

The code snippet below shows how simple it is to build and run xv6. The whole process takes 2 minutes on my machine.

# This is for Ubuntu/Debian based Linux host machines.
sudo apt install git make gcc qemu
git clone https://github.com/mit-pdos/xv6-public
cd xv6-public
make
make qemu

It would also be very interesting to experiment whether it is possible to boot xv6 from inside MLL, probably by using kexec-tools or by any other means.