Add build instructions for multiple platforms

Added detailed build instructions for different platforms including prerequisites, build commands, output locations, and notes on icon placement.

BUILD.md

@@ -0,0 +1,54 @@
+# Build Instructions
+
+## Prerequisites
+
+Install dependencies:
+```bash
+npm install
+```
+
+## Building
+
+### Build for current platform:
+```bash
+npm run build
+```
+
+### Build for specific platform:
+
+**Windows:**
+```bash
+npm run build:win
+```
+
+**Linux:**
+```bash
+npm run build:linux
+```
+
+**macOS:**
+```bash
+npm run build:mac
+```
+
+### Build for all platforms:
+```bash
+npm run build:all
+```
+
+## Output
+
+Built executables will be in the `dist/` directory:
+
+- **Windows**: `Hytale F2P Launcher Setup.exe` (NSIS installer) and `Hytale F2P Launcher.exe` (portable)
+- **Linux**: `Hytale F2P Launcher.AppImage` and `Hytale F2P Launcher.deb`
+- **macOS**: `Hytale F2P Launcher.dmg` and `Hytale F2P Launcher.zip`
+
+## Notes
+
+- Icons need to be placed in `build/` directory:
+  - `icon.ico` for Windows
+  - `icon.png` for Linux
+  - `icon.icns` for macOS
+- To build for macOS on non-Mac systems, you'll need to run it on a Mac or use a CI/CD service
+