add dev container docs

.devcontainer/DEVCONTAINER.md

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+# Usage
+
+The Dockerfile will take care of installing all needed dependencies; all you should need to do once you enter the container is run `make` and then `./supersonic` to start it.  See the Linux/Ubuntu sections of BUILD.md for more details.  
+
+# VNC Access
+
+The dev container by default will open a desktop accessible via VNC at port :5901 or via web browser at :6080.
+
+You can use the VNC_RESOLUTION env var to adjust it if your monitors don't fit 1080p well
+
+supersonic can still be started via the terminal or debugger in VS Code; it will automatically open in the VNC desktop
+
+
+# Sound
+
+The container is configured to forward the default `/dev/snd` card so that supersonic can access it via the default ALSA driver.
+
+Depending on your host system, you may need to select a different sound card than what your system is using, or configure a dummy sound card, etc, as certain audio systems require exclusive access to the hardware device and don't like to share it with the container.
+
+If you encounter permission issues with accessing the device, make sure your user is in the `audio` group
+
+# Config
+
+The default config (`default-config.toml`) will be copied to the proper location in the home directory.  If you create a file in `.devcontainer/` named `custom-config.toml`, that will be copied instead.  It's in the `.gitignore`, so you can safely put whatever changes you want in there to point at your chosen servers, etc and persist them across dev container rebuilds.  
+
+# Future Improvements
+
+- Better sound integration to enable mixing with host system
+- X11 forwarding instructions