Add downloads badge

Author: mbilski

README.md

@@ -3,6 +3,7 @@
 [![status](https://github.com/cloudentity/oauth2c/workflows/build/badge.svg)](https://github.com/cloudentity/oauthc/actions)
 [![license](https://img.shields.io/badge/license-Apache--2.0-blue.svg)](https://www.apache.org/licenses/LICENSE-2.0.html)
 [![release](https://img.shields.io/github/release-pre/cloudentity/oauth2c.svg)](https://github.com/cloudentity/oauth2c/releases)
+[![downloads](https://img.shields.io/github/downloads/cloudentity/oauth2c/total)](https://github.com/cloudentity/oauth2c/releases)
 
 User-friendly command-line client for OAuth2