Added Zen Sounds as a productivity application in README.md

[arunyagoojar]

0. [x] I have read the Contribution Guidelines
1. [x] Project URL: https://github.com/arunyagoojar/Zen-Sounds-App
2. [x] Why should this project be added: It is a productivity application, which has a open source alternative to the portal application.
3. [x] End all descriptions with a full stop/period
4. [x] Entry is ordered alphabetically
5. [x] Appropriate icon(s) added if applicable (OSS, freeware)