add downloads shield

README.md

@@ -2,6 +2,7 @@
 
 [![Build status](https://github.com/solidiquis/erdtree/actions/workflows/ci.yml/badge.svg)](https://github.com/solidiquis/erdtree/actions)
 [![Crates.io](https://img.shields.io/crates/v/erdtree.svg)](https://crates.io/crates/erdtree)
+[![Crates.io](https://img.shields.io/crates/d/erdtree)](https://crates.io/crates/erdtree)
 
 A modern, vibrant, and multi-threaded file-tree visualizer and disk usage analyzer that respects hidden files and `.gitignore` rules by default i.e. the secret love child of [tree](https://en.wikipedia.org/wiki/Tree_(command)) and [du](https://en.wikipedia.org/wiki/Du_(Unix)).