Link up to the literature

Skuld is backed by some wicked tech. I've added README links to the relevant literature.

A couple of questions:

1. Instead of linking to Vogel's [post on Dynamo](http://www.allthingsdistributed.com/2007/10/amazons_dynamo.html), should I link directly to [the PDF](http://www.allthingsdistributed.com/files/amazon-dynamo-sosp2007.pdf)?
2. Is the viewstamped replication link your preferred reference, or is their something more up-to-date?