bug#39194: help for non-root users to start using

[Matt Wette]

This guix-1.0.1 on x86_64 Fedora 30.

After installing as root, it's not clear from the manual how users 
should start.
I found out "guix pull" is the right thing.
Maybe add that to the manual? (Or add a "guix init" command.)

Here is the error that I get w/o "guix pull":

[mwette@localhost ~]$ guix install hello
Backtrace:
           8 (primitive-load "/usr/local/bin/guix")
In guix/ui.scm:
  1813:12  7 (run-guix-command _ . _)
In ice-9/boot-9.scm:
    829:9  6 (catch _ _ #<procedure 7effb6272430 at guix/ui.scm:657?> ?)
    829:9  5 (catch _ _ #<procedure 7effb6272448 at guix/ui.scm:780?> ?)
In guix/scripts/package.scm:
   948:10  4 (_)
In guix/status.scm:
    768:4  3 (call-with-status-report _ _)
In guix/scripts/package.scm:
   956:14  2 (_)
In guix/build/syscalls.scm:
  1127:14  1 (call-with-file-lock/no-wait _ #<procedure 7effb42f5cd?> ?)
In ice-9/boot-9.scm:
    777:6  0 (throw "open-file" "~A: ~S" ("No such file or direc?" ?) ?)

ice-9/boot-9.scm:777:6: In procedure throw:
In procedure throw: Wrong type argument in position 1: open-file