seq comp documentation per #47

[lskatz]

#47

[lskatz]

I didn't see actual documentation on seqtk comp except on the issues page and so I am suggesting adding in some documentation. Thank you for your consideration!

[RPINerd]

I actually didn't even know about this feature of seqtk until seeing this pull request! It's definitely a function worth documenting.
I think the organization would benefit from using the pseudo-table type formatting built in to githubs markdown flavor, but it looks like the readme isn't using the github conventions anyways? I think having the pound signs at the start of each line doesn't quite fit with the rest of the readme style