Explain Visium and/or the data used in this documentation

[lcolladotor]

I think that it would be useful to explain to the reader where the data comes from instead of assuming that they know what we are talking about. So before https://github.com/LieberInstitute/VisiumImgProcessing/blob/705329a25b21f818e62a3606fec4a7af1f757192/Step1.Rmd#L3 I suggest describing what data was used for this documentation at https://github.com/LieberInstitute/VisiumImgProcessing/blob/705329a25b21f818e62a3606fec4a7af1f757192/index.Rmd#L11. You could link to say the image we have at https://github.com/LieberInstitute/spatialLIBD/blob/master/man/figures/paper_figure1.jpg and maybe re-use some of the documentation from https://github.com/LieberInstitute/spatialLIBD#study-design.

You could also make it more general and talk also more about Visium from 10X.