Convert Unix manual page from README.md for rpi-update

Here are some examples showing how to Convert manual page:
- https://github.com/sunaku/md2man#examples
> Try converting this example Markdown file into a UNIX manual page:
>
> ```
> md2man-roff EXAMPLE.markdown > EXAMPLE.1
> ```
>
> You can view the resulting UNIX manual page in your man(1) viewer:
> ```
> man -l EXAMPLE.1
> ```