Add donation link to README

Added a donation link to the README file.

Author: alaa-alsalehi

README.md

@@ -1,3 +1,13 @@
+<p align="center">
+  <a href="https://www.buymeacoffee.com/ruknsoftware" target="_blank">
+    <img
+      src="https://cdn.buymeacoffee.com/buttons/v2/default-red.png"
+      alt="Buy Me A Coffee"
+      style="height: 60px; width: 217px;"
+    />
+  </a>
+</p>
+
 # WalletE - Electronic Wallet System for ERPNext
 
 <div align="center">