docs: add s390x model conversion steps

Signed-off-by: Aaron Teo <[email protected]>

Author: taronaeo

docs/build-s390x.md

@@ -1,3 +1,6 @@
+> [!IMPORTANT]
+> This build documentation is specific only to IBM Z & LinuxONE mainframes (s390x). You can find the build documentation for other architectures: [build.md](docs/build.md).
+
 # Build llama.cpp locally (for s390x)
 
 The main product of this project is the `llama` library. Its C-style interface can be found in [include/llama.h](include/llama.h).
@@ -26,12 +29,24 @@ cmake --build build --config Release -j $(nproc)
 
 **Notes**:
 - For faster repeated compilation, install [ccache](https://ccache.dev/)
+- By default, VXE/VXE2 is enabled. To disable it (not recommended):
+
+    ```bash
+    cmake -S . -B build             \
+        -DCMAKE_BUILD_TYPE=Release  \
+        -DGGML_BLAS=ON              \
+        -DGGML_BLAS_VENDOR=OpenBLAS \
+        -DGGML_VXE=OFF
+    
+    cmake --build build --config Release -j $(nproc)
+    ```
+
 - For debug builds:
 
     ```bash
-    cmake -S . -B build \
-        -DCMAKE_BUILD_TYPE=Debug \
-        -DGGML_BLAS=ON \
+    cmake -S . -B build             \
+        -DCMAKE_BUILD_TYPE=Debug    \
+        -DGGML_BLAS=ON              \
         -DGGML_BLAS_VENDOR=OpenBLAS
 
     cmake --build build --config Debug -j $(nproc)
@@ -49,4 +64,50 @@ cmake --build build --config Release -j $(nproc)
     cmake --build build --config Release -j $(nproc)
     ```
 
+## Getting GGUF Models
+
+In order to run GGUF models, the model needs to be converted to Big-Endian. You can achieve this in three cases:
+
+1. Use pre-converted models verified for use on IBM Z & LinuxONE (easiest)
+
+You can find popular models pre-converted and verified at [s390x Ready Models](hf.co/collections/taronaeo/s390x-ready-models-672765393af438d0ccb72a08).
+
+These models and their respective tokenizers are verified to run correctly on IBM Z & LinuxONE.
+
+2. Convert safetensors model to GGUF Big-Endian directly (recommended)
+
+```bash
+python3 convert_hf_to_gguf.py \
+    --outfile model-name-be.f16.gguf \
+    --outtype f16 \
+    --bigendian \
+    model-directory/
+```
+
+For example,
+
+```bash
+python3 convert_hf_to_gguf.py \
+    --outfile granite-3.3-2b-instruct-be.f16.gguf \
+    --outtype f16 \
+    --bigendian \
+    granite-3.3-2b-instruct/
+```
+
+3. Convert existing GGUF Little-Endian model to Big-Endian
+
+```bash
+python3 gguf-py/gguf/scripts/gguf_convert_endian.py model-name.f16.gguf BIG
+```
+
+For example,
+```bash
+python3 gguf-py/gguf/scripts/gguf_convert_endian.py granite-3.3-2b-instruct-le.f16.gguf BIG
+mv granite-3.3-2b-instruct-le.f16.gguf granite-3.3-2b-instruct-be.f16.gguf
+```
+
+**Notes:**
+- The GGUF endian conversion script may not support all data types at the moment and may fail for some models/quantizations. When that happens, please try manually converting the safetensors model to GGUF Big-Endian via Step 2.
+
+