Revise README with project details and features

[atharvachavan289-gif]

Updated the README to provide a detailed overview of the Exchange Rate Calculator project, including key features, tech stack, and project structure.

Description

(Describe your project over here.)

Checklist

• I've been assigned an issue related to this PR.
• I've used beautifiers.
• I've added my Project's name and description to Index.md
• I've made a README.md file for my Project.
• The README.md file of my projrct contains Project title, Description, Use of project, Set up, Stack used and Output (Screenshots).

Related Issues or Pull Requests number

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[atharvachavan289-gif]

Added a clean, formatted README for the Exchange Rate Calculator project to help beginners understand the tech stack and how to run it locally.
This version strips out all the React, Vite, and Vercel specifics and perfectly matches the standard format expected in large "beginner-friendly" open-source repositories! Let me know if you need to add any specific API credits (like ExchangeRate-API) to it before you submit your PR.