Document how to .append() a directory

[joelmukuthu]

I think many are aware of archive.append(null, { name:'dir/' }), but since it's not documented it's hard for a newcomer to find out about it. Also, since the docs for .append(source, data) regard source as Buffer | Stream | String, it feels quite like going against the grain with null.

On this note, what do you think about adding some more explicit functions for this i.e. appendFile and appendDirectory that simply proxy to append?

I'd be happy to work on PRs for both of these if needed!