Add link to stream-reduce module

@dominictarr on #39:
> I'm not gonna merge this, because I now think having things as separate modules it better,
> This module is really just a historical thing now.
> 
> However, I would be more than happy to merge a readme change 
> that added a link to stream-reduce though.

I figured this is a good idea as probably a lot of people new to node streams use this package to learn about streams and the related module ecosystem.

It may also be a good idea to note that this package is not being developed any longer and why separate modules are better. It may also be good to mention that *through* and *map-stream* and the rest are separate modules themselves.