The guide

Everything you need to build an application on skm, in the order you will need it. No source reading required.

If you want to know how it works rather than how to use it, read ARCHITECTURE.md. If you are writing a second implementation of the wire, read PROTOCOL.md. Before you ship, read SECURITY.md — it is short, and it is the difference between an app that is end-to-end encrypted and one that says it is.