documentation: README.md - Document lowerCamelCase keys

[Insti]

In #987 it was decided that we should be consistent and use lowerCamelCase for all json key names. This needs to be documented in the README.md test data format section

It's currently mentioned in the comments within the example json, but I think it's worth explicitly mentioning outside of the example text that all keys should use that format.

Task

Update the "There are also some convention about expected that you must follow:" section of README.md to state that all json keys should be lowerCamelCase