Update faq.md with compression explanation

[Maarten-vd-Sande]

As briefly discussed on Gitter. A small explanation on how to enable gzip compression of the config.json.

[cmdcolin]

wonderful :) i didn't test this explicitly but it looks right, and it doesn't use mimemagic anywhere (which has caused much headache with bam and tabix, etc).... just nice, explicitly listed file types