Add docs

docs/usage.md

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+# Usage
+
+## GUI
+
+- When running the program manually without providing any arguments it will show a GUI.
+![gui](assets/gui.png "rpc-bridge GUI")
+- `Start` will start the service without installing itself.
+- `Install` will install the service.
+- `Remove` will uninstall the service.
+
+## Commands
+
+- `--help` Show help message
+	- This will show the help message
+
+- `--install` Install the service
+    - This will copy the binary to `C:\windows\bridge.exe` and register it as a service
+
+- `--uninstall` Uninstall the service
+    - This will remove the service and delete `C:\windows\bridge.exe`
+
+- `--steam` Reserved for Steam
+    - This will start the service and exit (used with `bridge.sh`)
+
+- `--no-service` Do not run as service
+    - (only for `--steam`)
+
+- `--service` Reserved for service
+    - Reserved
+
+- `--rpc <dir>` Set RPC_PATH environment variable
+    - This is used to specify the directory where `discord-ipc-0` is located