self explanatory fibonacci example program

[alv-around]

Hello!

I was checking #1156, and testing how will impact the cli and used this repo. While using it, I realized there are some gaps in repo. In my mind a user coming here should be run the example program with no issues.

For that I:

1. Provided a default openvm.toml data, so user can build out-of-the-box.
2. Extended README.md, so users can test locally if the basic cargo openvm build / run (:construction:) / keygen / prove work.

⚠️ Regarding cargo openvm run --input ... , I intend not to change the code in src/main.rs but I having trouble finding any working example to provide a value for n in stdin. I have grep OpenVm source but no luck so far. so I changed it provisionally to get it run. I will undo that change as soon I figure out how to "properly" format the hexstring.

[alv-around]

I am closing this PR, as I can not replicate the problems I encountered 2 weeks ago 🤷