Add instructions for enabling intellisense to dev environments

[hughrun]

From a closed PR in the main repository:

I think a better way to solve this ... is to use Visual Studio Code Dev Containers. I haven't previously been using this, but I've just set it up and it seems to work pretty well. Basically you install the plugin and then it mounts your local dev environment as a volume so that changes are made both inside and outside the running container. Since all the requirements are installed in the container, your VS Code can then go ahead and use intellisense for auto-completion and so on.

It requires a fairly simple .devcontainer/devcontainer.json file:

{
	"name": "Bookwyrm Development",
	"dockerComposeFile": ["../docker-compose.yml"],
	"service": "web",
	"workspaceFolder": "/app",
	 "forwardPorts": [1333, 8888]
}

Originally posted by @hughrun in bookwyrm-social/bookwyrm#2653 (comment)