Add more information on VScode README

[zezocas0]

It is possible to add the link directly from the vscode settings, you can add the raw link for the .css file. So there's no need to download the files at all.

In markdown.marp.themes is possible to add a item, like for example https://raw.githubusercontent.com/favourhong/Awesome-Marp/main/themes/am_red.scss.

[favourhong]

Thank you！but access to GitHub is slow in China, so using the GitHub URL directly is still rather limited.