Create macos_install.md

add MacOS Metal markdown install instructions

docs/macos_install.md

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+
+# llama-cpp-python - MacOS Install with Metal GPU
+
+
+**(1) Make sure you have xcode installed... at least the command line parts**
+```
+# check the path of your xcode install 
+xcode-select -p
+
+# xcode installed returns
+# /Applications/Xcode-beta.app/Contents/Developer
+
+# if xcode is missing then install it... it takes ages;
+xcode-select --install
+```
+
+**(2) Install the conda version for MacOS that supports Metal GPU**
+```
+wget https://github.com/conda-forge/miniforge/releases/latest/download/Miniforge3-MacOSX-arm64.sh
+bash Miniforge3-MacOSX-arm64.sh
+```
+
+**(3) Make a conda environment**
+```
+conda create -n llama python=3.9.16
+conda activate llama
+```
+
+**(4) Install the LATEST llama-cpp-python.. which, as of just today, happily supports MacOS Metal GPU**  
+    *(you needed xcode installed in order pip to build/compile the C++ code)*
+```
+pip uninstall llama-cpp-python -y
+CMAKE_ARGS="-DLLAMA_METAL=on" FORCE_CMAKE=1 pip install -U llama-cpp-python --no-cache-dir
+pip install 'llama-cpp-python[server]'
+
+# you should now have llama-cpp-python v0.1.62 installed
+llama-cpp-python         0.1.62      
+
+```
+
+**(4) Download a v3 ggml llama/vicuna/alpaca model**
+ - **ggmlv3**
+ - file name ends with **q4_0.bin** - indicating it is 4bit quantized, with quantisation method 0
+
+https://huggingface.co/vicuna/ggml-vicuna-13b-1.1/blob/main/ggml-vic13b-q4_0.bin
+https://huggingface.co/vicuna/ggml-vicuna-13b-1.1/blob/main/ggml-vic13b-uncensored-q4_0.bin
+https://huggingface.co/TheBloke/LLaMa-7B-GGML/blob/main/llama-7b.ggmlv3.q4_0.bin
+https://huggingface.co/TheBloke/LLaMa-13B-GGML/blob/main/llama-13b.ggmlv3.q4_0.bin
+
+
+**(6) run the llama-cpp-python API server with MacOS Metal GPU support**
+```
+# config your ggml model path
+# make sure it is ggml v3
+# make sure it is q4_0
+export MODEL=[path to your llama.cpp ggml models]]/[ggml-model-name]]q4_0.bin
+python3 -m llama_cpp.server --model $MODEL  --n_gpu_layers 1
+```
+
+***Note:** If you omit the `--n_gpu_layers 1` then CPU will be used*
+
+