doc: Improve "Installing Guix from Guix" section.

Reported by Mark H. Weaver <mhw@netris.org>.

* README (Installing Guix from Guix): Explicitly mention $PATH
  separately.  Mention $ACLOCAL_PATH, not $ACLOCAL.  Give the exact
  command to install the dependencies.  Remove mention of
  $GUIX_LD_WRAPPER_ALLOW_IMPURITIES, which is no longer needed.