add a screenshot.

os/README.md

@@ -4,6 +4,8 @@ inspired by [Julia Evan's blog post](http://jvns.ca/blog/2013/11/26/day-34-the-t
it clears the screen and then prints some text. exciting!
![screenshot of it running](./screenshot.png)
## How to run it
1. install `nasm` and `qemu`