Add badges

Author: jeromewu

README.md

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 FFmpeg.wasm Core
 ================
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+[![Node Version](https://img.shields.io/node/v/@ffmpeg/ffmpeg.svg)](https://img.shields.io/node/v/@ffmpeg/ffmpeg.svg)
 ![FFmpeg.wasm Core](https://github.com/ffmpegwasm/ffmpeg.wasm-core/workflows/FFmpeg.wasm/badge.svg?branch=n4.3.1-wasm)
+![npm (tag)](https://img.shields.io/npm/v/@ffmpeg/core/latest)
+[![Maintenance](https://img.shields.io/badge/Maintained%3F-yes-green.svg)](https://github.com/ffmpegwasm/ffmpeg.wasm-core/graphs/commit-activity)
+[![Downloads Total](https://img.shields.io/npm/dt/@ffmpeg/core.svg)](https://www.npmjs.com/package/@ffmpeg/core)
+[![Downloads Month](https://img.shields.io/npm/dm/@ffmpeg/core.svg)](https://www.npmjs.com/package/@ffmpeg/core)
 
 This is the core part of FFmpeg.wasm where we transpile C/C++ code of FFmpeg to JavaScript/WebAssembly code. It is still very experimental (and slow), but shows the possibilities of using FFmpeg purely in the browser.