docs: add doc for windows service

conf/windows/README.md

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+## Run `frp` as Windows service
+
+1. Download [winsw](https://github.com/winsw/winsw/releases)(`WinSW.NET2.exe` means need `.NET 2.0` runtime,and so on),Rename it to `frpc-service.exe` and `frps-service.exe` .
+2.  Put `frpc-service.xml` and `frpc-service.xml` to same directory as `frp*-service.exe` .
+3. Default location of frp is `C:\frp`, you can edit the xml config file.
+
+
+
+install  service 
+
+```shell
+# for frp client
+frpc-service.exe install 
+# for frp server
+frps-service.exe install 
+```
+
+> You will see frp service in windows service manager
+
+
+
+uninstall  service 
+
+```shell
+# for frp client
+frpc-service.exe uninstall 
+# for frp server
+frps-service.exe uninstall 
+```
+
+
+
+
+
+## 以Windows服务的方式运行`frp`
+
+1. 下载[winsw](https://github.com/winsw/winsw/releases)(它有多个运行时版本,`WinSW.NET2.exe` 的意思是需要安装 `.NET 2.0` ),然后将其分别重命名为两个文件: `frpc-service.exe` 和`frps-service.exe` .
+2.  将`frpc-service.xml` 和 `frpc-service.xml` 放到 `frp*-service.exe` 相同的目录下.
+3. 默认的frp安装目录是 `C:\frp`, 你可以在xml配置文件中修改。
+
+
+
+创建服务 
+
+```shell
+# for frp client
+frpc-service.exe install 
+# for frp server
+frps-service.exe install 
+```
+
+> 服务创建后你在Windows的服务管理器里面就能看见frp服务了
+
+
+
+服务卸载
+
+```shell
+# for frp client
+frpc-service.exe uninstall 
+# for frp server
+frps-service.exe uninstall 
+```
+
+
+

conf/windows/frpc-service.xml

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+<service>
+  <!-- ID of the service. It should be unique across the Windows system-->
+  <id>frpc</id>
+  <!-- Display name of the service -->
+  <name>frp client</name>
+  <!-- Service description -->
+  <description>Service for frp client</description>
+  <!-- frp install dir -->
+  <workingdirectory>C:\frp</workingdirectory>
+  <!-- frps.exe -->
+  <executable>C:\frp\frpc.exe</executable>
+  <!-- args -->
+  <startarguments>-c frpc.ini</startarguments>
+</service>

conf/windows/frps-service.xml

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+<service>
+  <!-- ID of the service. It should be unique across the Windows system-->
+  <id>frps</id>
+  <!-- Display name of the service -->
+  <name>frp server</name>
+  <!-- Service description -->
+  <description>Service for frp server</description>
+  <!-- frp install dir -->
+  <workingdirectory>C:\frp</workingdirectory>
+  <!-- frps.exe -->
+  <executable>C:\frp\frps.exe</executable>
+  <!-- args -->
+  <startarguments>-c frps.ini</startarguments>
+</service>