Update README to better reflect purpose of this repo

[matthewmturner]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
(This section helps Arrow developers understand the context and why for this feature, in addition to the what)

Describe the solution you'd like
A clear and concise description of what you want to happen.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[Dandandan]

The benefit of keeping the current structure is that

• We can keep it up to data with upstream datafusion
• We run the entire datafusion test suite
• We can eventually merge this back into datafusion

The current tokomak README.md is located here:
https://github.com/datafusion-contrib/datafusion-tokomak/tree/main/tokomak

[matthewmturner]

Makes sense, thanks!

[Dandandan]

I wanted to suggest to keep the issue open. Maybe there is a smart way of dealing with this, such as making the default branch not the development branch or doing something smart with the README location.
Suggestions welcome :).

[matthewmturner]

Ok got it - ill think on it.