docs: add Open VSX badges (#218)

Author: JounQin

README.md

@@ -1,9 +1,11 @@
 # [Visual Studio Code](https://code.visualstudio.com) extension for [MDX][]
 
 [![GitHub Actions](https://github.com/mdx-js/vscode-mdx/workflows/CI/badge.svg)](https://github.com/mdx-js/vscode-mdx/actions/workflows/ci.yml)
-[![Visual Studio Marketplace Version](https://img.shields.io/visual-studio-marketplace/v/unifiedjs.vscode-mdx)](https://marketplace.visualstudio.com/items?itemName=unifiedjs.vscode-mdx)
 [![GitHub release](https://img.shields.io/github/release/mdx-js/vscode-mdx)](https://github.com/mdx-js/vscode-mdx/releases)
+[![Visual Studio Marketplace Version](https://img.shields.io/visual-studio-marketplace/v/unifiedjs.vscode-mdx)](https://marketplace.visualstudio.com/items?itemName=unifiedjs.vscode-mdx)
 [![Visual Studio Marketplace Downloads](https://img.shields.io/visual-studio-marketplace/d/unifiedjs.vscode-mdx)](https://marketplace.visualstudio.com/items?itemName=unifiedjs.vscode-mdx)
+[![Open VSX Version](https://img.shields.io/open-vsx/v/unifiedjs/vscode-mdx)](https://open-vsx.org/extension/unifiedjs/vscode-mdx)
+[![Open VSX Downloads](https://img.shields.io/open-vsx/dt/unifiedjs/vscode-mdx)](https://open-vsx.org/extension/unifiedjs/vscode-mdx)
 [![Language grade: JavaScript](https://img.shields.io/lgtm/grade/javascript/g/mdx-js/vscode-mdx.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/mdx-js/vscode-mdx/context:javascript)
 
 [![Conventional Commits](https://img.shields.io/badge/conventional%20commits-1.0.0-yellow.svg)](https://conventionalcommits.org)