fix: add missing rustpix-gui README.md for maturin wheel build

The pyproject.toml referenced README.md but the file did not exist,
causing maturin to fail on both Linux and macOS GUI wheel builds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: KedoKudo

rustpix-gui/README.md

@@ -0,0 +1,38 @@
+# rustpix-gui
+
+GUI application for rustpix pixel detector processing.
+
+## Installation
+
+### macOS (Homebrew)
+
+```bash
+brew tap ornlneutronimaging/rustpix
+brew install --cask rustpix
+```
+
+### pip
+
+```bash
+pip install rustpix-gui
+rustpix-gui
+```
+
+### From Source
+
+```bash
+cargo run --release -p rustpix-gui
+```
+
+## Features
+
+- Interactive file loading (open or drag-and-drop TPX3 files)
+- Real-time 2D detector map visualization
+- ABS, DBSCAN, and Grid clustering algorithms
+- Parameter tuning with immediate visual feedback
+- ROI selection for focused analysis
+- Export to HDF5 (NeXus), HDF5 (SNS NXsnsevent), TIFF, and CSV
+
+## Documentation
+
+See the [full documentation](https://ornlneutronimaging.github.io/rustpix/gui/).