Add BUILD_WinArm64.md

src/BUILD_WinArm64.md

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+# How to build and install SoftEther VPN on Windows ARM64
+
+This document describes how to build SoftEther VPN for Windows ARM64 and how to install the VPN Client and Neo6 virtual network adapter on Windows on ARM devices.
+
+
+## Requirements
+
+
+- Build host: Windows x64
+
+- Target device: Windows 10 / Windows 11 ARM64
+
+
+## Building 
+ 
+    **Notes before building**: ARM64 builds are cross-compiled from an x64 Windows host. An existing x64-native build is required to generate hamcore.se2.
+1. Follow [BUILD_WINDOWS.md](BUILD_WINDOWS.md##Building)
+
+1. Build x64 (Native): From the build menu, select x64-on-x64. Complete the build successfully. This build is required to generate shared resources
+
+1. Build ARM64 (Cross-Compiled): From the same build menu, select arm64-on-x64. 
+Build the ARM64 version of SoftEther VPN.
+
+1. Building the Neo6 Virtual Network Adapter (ARM64)
+
+    Open the following project in Visual Studio:
+    ```
+    .\src\Neo6\Neo6.vcxproj 
+    ```
+   
+    SoftEther VPN Client uses the Neo6 virtual network adapter.
+
+
+    Driver Output Files
+    The ARM64 driver package includes:
+    ```
+    Neo6_arm64_VPN.sys
+    Neo6_arm64_VPN.inf
+    ```
+    Driver Signing and Installation (Windows ARM64)
+    ```
+    Enable test-signing mode: bcdedit /set testsigning on
+    Reboot the system.
+    Testing signing:
+    Install the Neo6 ARM64 driver.
+    ```
+# Summary
+
+SoftEther VPN can be cross-compiled for Windows ARM64 on an x64 host
+VPN Client works natively on Windows on ARM
+Neo6 ARM64 driver requires Microsoft signing for production use
+Test-signing is suitable for local development only