doc: invoke useradd with -m flag, to create homedir
doc: invoke useradd with -m flag, to create homedir

Change-Id: Ib0dcbba5d42db2277c2d57fdf40b1f36958faab1
Reviewed-on: https://upspin-review.googlesource.com/8384
Reviewed-by: Michael Stapelberg <97e0203ad32d728c266b6422d4bd97c18b9b600f@google.com>
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/upspin,upspin/upspin