Consider explaining "searching for packages" more detailed

[manuliner]

Observations
I am doing the nixOS tutorial. A very important part of creating nix.files is the search for missing dependecies. It is just mentioned in a small note that this is the solution for the problem in the tutorial.

Problem

Left out how to find X11/Xlib.h throuhg nix-locate etc.

Approaches
Small Tutorial for a better understanding how to find this dependcies with nix-tools would be nice

[fricklerhandwerk]

fixed by #887