add demo video in readme

README.md

@@ -33,7 +33,12 @@ Cross-platform hassle-free file sharing with AES-256 encryption made with Flutte
 
 <p align="center"><img src="assets/header.png" alt="Open source easy file sharing for everyone." width="1000px" /></p>
 
+<details><summary><strong>Demo</strong></summary>
+
+https://user-images.githubusercontent.com/42910433/143038817-cb935815-aea7-41c1-8b56-131cb99b0a20.mp4
 
+</details>
+
 
 ## Built With
 
@@ -142,4 +147,4 @@ Distributed under the GPL-3.0 License. See `LICENSE` for more information.
   <img src="https://contributors-img.web.app/image?repo=odinapp/odin" />
 </a>
 
-### If you made it here, thanks for your support. You can show more support by starring this repo. See ya! 👋
+### If you made it here, thanks for your support. You can show more support by starring this repo. See ya! 👋