Add example token generate for delta-generator

[btkostner]

This was one of the things that took me a bit of time to figure out looking at the code when setting up a repository. Hopefully this helps other people get setup faster.

Two other things I'd also like to add to the README (but will make other PRs for)

• Talking about the delta-generator bin
• Repository configuration, subsets, and the usual way repos are broken apart