config.toml documentation

[HannoSpreeuw]

In the more general sense we should provide directions for users that want to work with a config.toml.

More specifically, we should clarify, i.e. document, as part of the README, that they should include a tool.pyse.image and a tool.pyse.export section, as the config.toml in the test/data directory.

Otherwise they may have a hard time to figure that out.