#2756 add documentation on windows proxy service

Author: totaam

docs/Usage/Windows-Service.md

@@ -0,0 +1,41 @@
+# Windows System Service
+
+This service is only included in full builds.
+
+
+The [`Xpra-Service.cpp`](https://github.com/Xpra-org/xpra/blob/master/packaging/MSWindows/service/Xpra-Service.cpp)
+is compiled into `Xpra-Service.exe` using `vscode` or just via `msbuild` during the build.
+
+This service can be installed by running `./Xpra-Service.exe install`,
+then it can be managed from the GUI `services.msc` tool.
+It is not configured to start automatically. \
+Use `./Xpra-Service.exe uninstall` to unregister it.
+
+The xpra `.exe` installer should have created the registry key `HKLM\Software\Xpra\InstallPath`,
+typically pointing to `C:\Program Files\Xpra`.
+
+The `start` action for this service will run `Xpra-Proxy.exe start` from this `InstallPath`.
+
+The `Xpra-Proxy.exe` scripts is a simple delegation wrapper ([source](https://github.com/Xpra-org/xpra/blob/master/xpra/platform/win32/scripts/proxy.py))
+packaged as a GUI application which just calls
+[`xpra.platform.win32.service`](https://github.com/Xpra-org/xpra/blob/master/xpra/platform/win32/service.py).
+
+This service just starts an xpra proxy server using the hard-coded arguments:
+```shell
+Xpra proxy --bind-tcp=0.0.0.0:14500,auth=sys,client-username=true,verify-username=true
+```
+
+The proxy will log to a file.
+The exact path of this log file can be obtained by querying the proxy server:
+```shell
+Xpra_cmd info tcp://localhost:14500/ | grep log-file
+```
+Normally, it should be `C:\Windows\System32\config\systemprofile\AppData\Local\Xpra\System-Proxy.log`
+
+---
+
+Work in progress: it isn't entirely clear which API the proxy
+is meant to use to start a new session, station and desktop once users
+have authenticated.
+At time of writing, the proxy server can only mediate for existing sessions,
+and only for `Administrators`.