docs: Add GitHub downloads badge to README

Author: inureyes

README.md

@@ -2,6 +2,7 @@
 
 [![Crates.io version](https://img.shields.io/crates/v/bssh.svg?style=flat-square)](https://crates.io/crates/bssh)
 [![Crates.io downloads](https://img.shields.io/crates/d/bssh.svg?style=flat-square)](https://crates.io/crates/bssh)
+![GitHub Downloads](https://img.shields.io/github/downloads/lablup/bssh/total?style=flat-square&label=downloads)
 ![CI](https://github.com/lablup/bssh/workflows/CI/badge.svg)
 [![dependency status](https://deps.rs/repo/github/lablup/bssh/status.svg)](https://deps.rs/repo/github/lablup/bssh)