Add missing build/setup instructions to README.md

[Varshad-Potle]

File: README.md
Section: Getting Started / Build instructions
Problem: The documentation references gulp build, but gulp is no longer used in the project. Running this command fails.
Steps to reproduce:

1. Clone the repo.
2. Run gulp build as instructed.
3. Observe the error (command not found).
   Suggested fix: Replace gulp build with npm run build in README.md, based on the current build scripts in package.json.
   Environment tested: Node.js v20.x