Adds --output to the docs

[S1SYPHOS]

It's not much, but it's something 🥉

One thing though, maybe include the new path in the output, eg:

> glyphhanger --subset=*.ttf --output=some/other/path

Subsetting LatoLatin-Regular.ttf to some/other/path/LatoLatin-Regular-subset.woff (was 145.06 KB, now 2.88 KB)
Subsetting LatoLatin-Regular.ttf to some/other/path/LatoLatin-Regular-subset.woff2 (was 145.06 KB, now 2.24 KB)

.. great software, thank you very much!

[S1SYPHOS]

Anything you need to get this party started? 🎂