Update Readme with Link to HLSL to GLSL guide

[hawkerm]

When working on adding a new shader from ShaderToys in #147.

I was a bit confused on how to map some of the GLSL functions to the HLSL equivalents used in ComputeSharp.

@Sergio0694 pointed out a good blog he uses: https://anteru.net/blog/2016/mapping-between-HLSL-and-GLSL/

Would be handy to add this to the readme for others that may need it in the future.