Write a readme on Multi-GPU usage in llama.cpp (#22729)

* Write a readme on Multi-GPU usage in llama.cpp

* Apply suggestions from code review

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

* Address review comments

* Apply suggestions from code review

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

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Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

README.md

@@ -529,6 +529,7 @@ To learn more about model quantization, [read this documentation](tools/quantize
 - [How to build](docs/build.md)
 - [Running on Docker](docs/docker.md)
 - [Build on Android](docs/android.md)
+- [Multi-GPU usage](docs/multi-gpu.md)
 - [Performance troubleshooting](docs/development/token_generation_performance_tips.md)
 - [GGML tips & tricks](https://github.com/ggml-org/llama.cpp/wiki/GGML-Tips-&-Tricks)