Document demonstration

Make a short document documentation/DEMO.md showing how to make a demonstration. It can be a quite simple file containing:

• a user-friendly text describing the intent, e.g. adding 2 files on a 5-node swarm, getting a file, then corrupting a few shards, and getting the file again because enough shards are correct. This can be illustrated with a picture.
• the list of raw commands, with comments