Updated README to improve formatting and add direct download links for easier access

[SimonEast]

Hi ICSM team,

I came across this repo looking for some spreadsheets to do grid conversions, and thought it could use a few formatting improvements. I also added direct download links to each of the files listed, as clicking on the files within Github is a bit confusing as you have to click a further download button to actually get the file. These links go direct.

Hopefully this is useful.

If you approve of my changes there should be a link to "Merge" my updates into your main repository.

Cheers, Simon