[doc] Arch Linux installation: modified the command for user creation so it works on both Debian and Arch

FW requires Python>=3.5 ; only Python 3.4 is packaged in Debian 8.

fix #300

Without that, starting from a debian « naked » system,  it will fail at the `pip install wheel` stage.

rmdir will fail on that non-empty dir

run makemigrations yourself