Install doc (binary installation)

[Pjotr Prins]

I just went through 

  https://www.gnu.org/software/guix/manual/html_node/Binary-Installation.html

And it works well enough.

Two minor niggles:

1. Can we inline section 4 on creating users and build group? I think
   people easily miss this step now, creating problems a bit later.

2. Add 'guix pull' step. I am no fan of the latter, but doing a 'guix
   package -i guix' installs an older version which we may want to
   avoid.

Otherwise it is nice as ever. HNY.

Pj.