add downloads badge again

Author: jonaspleyer

README.md

@@ -3,6 +3,7 @@
 ![Test](https://img.shields.io/github/actions/workflow/status/brendanzab/approx/test.yml?style=flat-square&label=Test)
 [![Crate](https://img.shields.io/crates/v/approx.svg?style=flat-square)](https://crates.io/crates/approx)
 [![License Apache](https://img.shields.io/badge/License-Apache%202.0-brightgreen.svg?style=flat-square)](https://opensource.org/licenses/Apache-2.0)
+[![Downloads](https://img.shields.io/crates/d/approx?style=flat-square)](https://crates.io/crates/approx)
 [![Docs](https://img.shields.io/docsrs/approx?style=flat-square)](https://docs.rs/approx)
 [![Minimum rustc version](https://img.shields.io/badge/rustc-1.36+-lightgray.svg?style=flat-square)](https://github.com/brendanzab/approx#rust-version-requirements)