Add the quota command

[waldyrious]

We currently have at least one command per letter of the alphabet in the common folder, except for Q. I figured a good candidate would be quota, but it seems to have different incarnations depending on the system. If anyone's familiar with it, please start a page so we can cover the whole alphabet!

Also, check out this list: https://github.com/tldr-pages/tldr/wiki/Pages-plan (Update 2017-09-15: this is now tracked in #1070.)

[leostera]

Don't we have quit ? D:

[waldyrious]

@Ostera is that an actual command? I can't seem to find any reference to it. Or am I missing a joke somewhere?

[agnivade]

Oh we have a pages plan ! Gosh .. didn't realise so many commands were left out. Brace yourselves, PRs are coming !