docs: add preview to README

Author: ankddev

README.md

@@ -7,6 +7,9 @@
     <a href="https://crates.io/crates/envfetch"><img src="https://img.shields.io/crates/d/envfetch" alt="crates.io downloads"/></a>
     <a href="https://crates.io/crates/envfetch"><img src="https://img.shields.io/crates/v/envfetch" alt="crates.io version"/></a>
 </div>
+<div align="center">
+    <img src="https://github.com/user-attachments/assets/261ea1fd-438a-40b0-847d-6a460b7a30a9" />
+</div>
 
 # Features
 - [x] Print all environment variables