integrations: document steps to build GDAL from source on Windows

Author: Grok Compression

integrations/gdal/README.md

@@ -19,4 +19,43 @@ GDAL_ENABLE_DRIVER_JP2GROK  set to on
 
 CMAKE_PREFIX_PATH
 /home/FOO/bin/grok
+```
+
+# Install GDAL from source on Windows (with Visual Studio 2026)
+
+1. Install miniconda
+
+2. Open a conda prompt (not PowerShell)
+
+## Install environment
+
+```
+conda create -n gdal-src python=3.11 cmake swig numpy pytest -c conda-forge
+conda activate gdal-src
+conda install -c conda-forge --only-deps libgdal-core
+conda install -c conda-forge openjpeg ninja
+```
+
+## Clone GDAL
+
+```
+mkdir src
+cd src
+git clone https://github.com/OSGeo/gdal.git
+cd gdal
+git checkout v3.10.2   # or v3.11.x / main for latest
+```
+
+## Build GDAL with Ninja
+
+```
+mkdir build
+cd build
+
+call "C:\Program Files\Microsoft Visual Studio\18\Community\VC\Auxiliary\Build\vcvarsall.bat" x64
+
+cmake .. -G Ninja -DCMAKE_INSTALL_PREFIX="%CONDA_PREFIX%" -DPython3_EXECUTABLE="%CONDA_PREFIX%\python.exe" -DGDAL_USE_EXTERNAL_LIBS=ON -DGDAL_BUILD_OPTIONAL_DRIVERS=ON -DOGR_BUILD_OPTIONAL_DRIVERS=ON -DBUILD_TESTING=OFF
+
+cmake --build . --config Release --parallel 16
+cmake --install . --config Release
 ```