Add a README link under related related projects for gpu.cpp under WebGPU C++

[austinvhuang]

Add a README link under related related projects for gpu.cpp under WebGPU C++.

For background - gpu.cpp is a new project I've been working on. It's a small library for writing portable GPU code in C++ that works across vendors by using a native (ie non-browser) WebGPU implementation (Dawn from Google). It aims to be a general-purpose library, but will also port the dev/cuda kernels to make llm.c kernels broadly available as WebGPU kernels using the WebGPU Shading Language (WGSL).

Some relevant discussion here: https://github.com/karpathy/llm.c/discussions/670