In f78bda9f, recommended configuration files were added to be used as
basis for setting up [Remote Development using docker containers][1] in
VSCode. Apparently, I had forgotten to commit the corresponding
`README.md` file.

This commit adds `.devcontainer/README.md` with more info on Remote
Development in VSCode.

[1]: https://code.visualstudio.com/docs/remote/containers

PR Close #33790