Add link to VS Code Marketplace

[Marcono1234]

In this repository here, could you please add a link to the VS Code Marketplace entry (I assume it is https://marketplace.visualstudio.com/items?itemName=MermaidChart.vscode-mermaid-chart)? That would make it easier to find there.

For example

• Add a link at the "About" section of this GitHub repo
• Add a link / badge in the README