Add additional explanation in "Managing the Configuration with Git" section

[Nyk Ma]

Thank you for this awesome guide. IMHO there is one more very important piece of information that needs to be added to this section:

After you git init the configuration folder, you have to git add all the files into the staged status before you do the nixos-rebuild switch, otherwise you will see a very unclear error message.

Thank you.

[Ryan Yin]

Updated, thanks for the feedback!