Adds instructions on README.md to use the git-graph.toml config file

[KarlHeitmann]

It took me some time to figure out how to use the git-graph.toml file with the default configuration for --model <model>, I've had to read the code to figure out where to put the config file.

This PR provides a sample git-graph.toml.sample file and instructions where to copy and paste it.

[mlange-42]

This may not be needed. See this suggestion: https://github.com/mlange-42/git-graph/pull/71#issuecomment-1623253021