Document include_extensions

[ToxicFrog]

It's a really useful feature that doesn't show up in --help or the man page at all.

It would also be worth mentioning that both include_extensions and hide_extensions can be specified multiple times and will be combined, which is necessary when specifying them in the fstab as you can't use commas to separate the extensions there.

[nschlia]

That probably escaped me... I'll fix that.