Create README.md for browser extension

[vudangbaovy]

✨ Create README.md for browser extension

📓 Referenced Issue

N/A - I found a need for there to be documentation on the browser extension specifically.

ℹ️ About the PR

Adding a README for the Responsively Helper extension to document its use cases, past code changes and its maintenance in the present.

🖼️ Testing Scenarios / Screenshots

N/A

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