Add example data or description of input data format

[shsiena]

In the current state of this repo, dependency issues aside, it isn't clear how the input data should be formatted

the only instructions are Usage: ray_voxel <metadata.json> <image_folder> <output_voxel_bin>, but it isn't specified what format metadata.json should take.

If possible, a set of example data should be included, but if not, at least the structure of metadata.json should be given.