Add command to switch to root

Installation guide assumed you are root but didn't explain how to get there. And it's not obvious.

Changed files
+8 -2
nixos
doc
manual
from_md
installation
installation
+4 -1
nixos/doc/manual/from_md/installation/installing.chapter.xml
···
<para>
You are logged-in automatically as <literal>nixos</literal>. The
<literal>nixos</literal> user account has an empty password so you
-
can use <literal>sudo</literal> without a password.
+
can use <literal>sudo</literal> without a password:
</para>
+
<programlisting>
+
$ sudo -i
+
</programlisting>
<para>
If you downloaded the graphical ISO image, you can run
<literal>systemctl start display-manager</literal> to start the
+4 -1
nixos/doc/manual/installation/installing.chapter.md
···
The NixOS manual is available by running `nixos-help`.
You are logged-in automatically as `nixos`. The `nixos` user account has
-
an empty password so you can use `sudo` without a password.
+
an empty password so you can use `sudo` without a password:
+
```ShellSession
+
$ sudo -i
+
```
If you downloaded the graphical ISO image, you can run `systemctl
start display-manager` to start the desktop environment. If you want