Add manual installation instructions (mostly for troubleshooting)

MANUAL-INSTALL.md

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+<!-- vim: set tw=120: -->
+
+# Manual Build and Install
+
+- Install the virtual audio device `Background Music Device.driver` to `/Library/Audio/Plug-Ins/HAL`.
+
+  ```shell
+  sudo xcodebuild -project BGMDriver/BGMDriver.xcodeproj \
+                  -target "Background Music Device" \
+                  RUN_CLANG_STATIC_ANALYZER=0 \
+                  DSTROOT="/" \
+                  install
+  ```
+- Install the XPC helper.
+
+  ```shell
+  sudo xcodebuild -project BGMApp/BGMApp.xcodeproj \
+                  -target BGMXPCHelper \
+                  RUN_CLANG_STATIC_ANALYZER=0 \
+                  DSTROOT="/" \
+                  INSTALL_PATH="$(BGMApp/BGMXPCHelper/safe_install_dir.sh)" \
+                  install
+  ```
+- Install `Background Music.app` to `/Applications` (or wherever).
+
+  ```shell
+  xcodebuild -project BGMApp/BGMApp.xcodeproj \
+             -target "Background Music" \
+             RUN_CLANG_STATIC_ANALYZER=0 \
+             DSTROOT="/" \
+             install
+  ```
+- Restart `coreaudiod`: <br>
+  (Audio will stop working until the next step, so you might want to pause any running audio apps.)
+
+  ```shell
+  sudo launchctl kill -15 system/com.apple.audio.coreaudiod
+  ```
+  or, if that fails
+
+  ```shell
+  sudo killall coreaudiod
+  ```
+- Run `Background Music.app`.
+
+