docs: update README, add section for how to install

[cadamsdev]

🔗 Linked issue

n/a

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR updates the README to add a section for how to install the package. This is useful so devs can look at the modules page
https://nuxt.com/modules/image
and know how to install it without having to click a separate link to get to the documentation.

This is also consistent with other official nuxt modules that have an Installation section in the README.

• https://nuxt.com/modules/seo#installation
• https://nuxt.com/modules/icon#setup-%EF%B8%8F
• https://nuxt.com/modules/i18n#install
• https://nuxt.com/modules/ui#installation

Related PR

• docs: add Installation section in README fonts#677

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commit: 2fda10e