Docs: Add minimal “Quick Start” section to README (non-breaking)

[Chrilleweb]

Describe the change

Add a short Quick Start section to help new users get started faster.
This is a documentation-only change.

What to change

In README.md, add a Quick Start section near the top (after the clean/successful scan):

Quick Start

npm install dotenv-diff

yarn add dotenv-diff

pnpm add dotenv-diff

Scope

• Documentation only
• No code changes
• No behavior changes

Additional context

Many users skim README files. A quick start reduces friction and improves adoption.