Documentation for reader::ReadIndices

[JulianKnodt]

I think it would be useful to note that the reader::ReadIndices returns a flat set of indices, and parsing those indices into tris or lines should be done by checking the mode. It may also be useful to add a utility which converts the flat iterator into an iterator over arrays of [u32; 3] (altho array chunks does this).

I can also contribute this if you think it's good.