add standalone instructions to readme

[obbardc]

normally fakemachine is used for debos, but can be used on it's own to replace software like fakeroot:

export GOPATH=~/go
export PATH=$PATH:~/go/bin
go get -u github.com/go-debos/fakemachine/cmd/fakemachine

$ fakemachine -h
Usage:
  fakemachine [OPTIONS]

Application Options:
  -v, --volume=      volume to mount
  -i, --image=       image to add
  -m, --memory=      Amount of memory for the fakemachine in megabytes
  -c, --cpus=        Number of CPUs for the fakemachine
  -s, --scratchsize= On-disk scratch space size (with a unit suffix, e.g. 4G); if unset, memory backed scratch space is used
      --show-boot    Show boot/console messages from the fakemachine

Help Options:
  -h, --help         Show this help message

$ fakemachine echo test
Running echo test
test
Powering off.