docs: add project icon and status badges to the README

Author: 1e1

README.md

@@ -1,5 +1,15 @@
+<div align="center">
+
+<img src="src/icon.png" alt="RPN app icon" width="64" height="64">
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 # RPN for NumWorks
 
+[![GitHub release](https://img.shields.io/github/v/release/1e1/numworks-RPN?style=flat-square)](https://github.com/1e1/numworks-RPN/releases)
+[![GitHub commit activity](https://img.shields.io/github/commit-activity/m/1e1/numworks-RPN?style=flat-square&color=brightgreen)](https://github.com/1e1/numworks-RPN/commits/main)
+[![License](https://img.shields.io/github/license/1e1/numworks-RPN?style=flat-square)](https://github.com/1e1/numworks-RPN/blob/main/LICENSE)
+
+</div>
+
 A **Reverse Polish Notation** (RPN) calculator as an external app for the
 [NumWorks](https://www.numworks.com) graphing calculator.