Create documentation

[dzufferey]

We need to create a proper documentation for:

• explain new features (round signature, controlling the runtime, etc.)
• a proper tutorial
• runtime options and performance consideration
• dealing with Byzantine faults (what guarantees does PSync provide, what is up to the user)
• dynamic group membership (integrating/removing processes while the system runs)
• integrating in a larger system (interface, out-of-band messages, sharing threads)
• ... what did I forgot ?