Add additional shader that always casts original shadows

[jawinn]

Added an additional shader called DitherTransparentOriginalShadow. Instead of a dithered shadow, the original shadow will always be cast regardless of the opacity. An additional node was added to the shader graph, using the technique referenced by Unity on this forum post, that sets IsShadowCaster to true: https://forum.unity.com/threads/alpha-clip-dither-and-keep-original-shadows.781412/#post-5212907

I also added a basic "Usage" section to the README. The README could use some more elaboration to add some of the relevant usage/implementation details found in the linked tutorial.