Document "xo"'s --not-first option

[philshafer]

See https://lists.freebsd.org/pipermail/freebsd-hackers/2018-September/053304.html

>"bhyve": {
>  "machines": {
>    "machine": {
>      "name": "test1",
>      "memory": "2GB"
>    }    "machine": {
>      "name": "test2",
>      "memory": "4GB"
>    }
>  }
>}

There's an "xo" option called "--not-first" for exactly these
situations; place that option on the second invocation so it
will know to emit the leading comma. This should be documented (#59).

[philshafer]

Added in 02caa91..638d6ba