document how to use with VS Code and VS Codium

[bpmct]

VS Code docs example: https://github.com/eclipse/openvsx/wiki/Using-Open-VSX-in-VS-Code

VS Codium docs example: https://github.com/VSCodium/vscodium/blob/master/DOCS.md#how-to-use-the-vs-code-marketplace

[jonathanalgar]

VS Code docs example: https://github.com/eclipse/openvsx/wiki/Using-Open-VSX-in-VS-Code

VS Codium docs example: https://github.com/VSCodium/vscodium/blob/master/DOCS.md#how-to-use-the-vs-code-marketplace

Thanks. Suggest to surface these links to the body of the README.