Extend readme with instructions for vscode

[mfs12]

Description

@mloidl wrote:

Did not try much except compilation and a few code changes to get a feeling.

• Code completion seems to work
• Compilation is straight forward once env.cmake is existent (otherwise you'll need to pass the device somewhere in the settings)
• Include files are all found

Tested on archlinux. Required dependencies (compiler, vscode-cmake-plugin, ...) were installed prior to my test. So for a new setup there might be some additional steps to do.