add llvm spir-v backend to readme (#37)

* add llvm spir-v backend to readme

Fixes #33 (converted to discussion since)

I'm not including VCC here, as I didn't understand enough from its homepage to write a proper comparison.

* Tweak wording

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Co-authored-by: Christian Legnitto <LegNeato@users.noreply.github.com>

Author: Ekleog

README.md

@@ -120,6 +120,7 @@ There are a few different components to this repo:
 
 Historical and other related projects for compiling Rust code to GPUs.
 
+- [SPIR-V LLVM backend](https://llvm.org/docs/SPIRVUsage.html) Compiles to OpenCL SPIR-V, which differs from Vulkan SPIR-V used by Rust GPU.
 - 2016: [glassful](https://github.com/kmcallister/glassful) Compiles a subset of Rust to GLSL.
 - 2017: [inspirv-rust](https://github.com/msiglreith/inspirv-rust) Experimental Rust to SPIR-V compiler.
 - 2018: [nvptx](https://github.com/japaric-archived/nvptx) Rust to PTX compiler.