fix output directory name and add docs

Having the two scripts expect the files to be in different directories caused some confusion, because if you modify the svg file and then regenerate the cursor files, not error would be thrown and the cursor files generated would be from the png files tracked by this repo, not by the newly rendered pngs.

As of the documentation, there's more room to improve the it(and building scripts), given the target audience includes designers that are not programmers.