add build documentation

BUILD.md

@@ -0,0 +1,36 @@
+# Build Instructions
+
+## Prerequisites
+
+### Windows
+- Rust toolchain (MSVC target)
+- Visual Studio Build Tools with C++ workload
+- Windows SDK
+
+Install via:
+```powershell
+rustup default stable-msvc
+```
+
+### Linux
+- Rust toolchain
+- GCC/Clang
+- libelf-dev (for eBPF)
+
+### macOS
+- Rust toolchain
+- Xcode Command Line Tools
+
+## Building
+
+```bash
+cargo build --release
+```
+
+## Running
+
+```bash
+cargo run --bin ghost-cli
+```
+
+Note: Requires elevated privileges for full process memory access.