docs(install): Enhance CLI installation guide with OS-specific steps and Arkade option

[davmano]

This PR enhances the README.md with a comprehensive Quick Start Installation Guide for faas-cli.
It introduces:

✅ Arkade-based installation (recommended)

🐧 Linux/macOS setup with root and non-root options

🍺 Homebrew installation for macOS

🪟 PowerShell instructions for Windows users

✅ Command to verify successful installation

🧪 First function deployment example for onboarding

Purpose & Benefits:

These changes aim to:

• 🚀 Improve onboarding experience for new users

• 🧭 Provide OS-specific clarity

• 🤝 Align with OpenFaaS documentation and community standards

✅ Checklist

• My changes follow the contributing guidelines

• I've tested the installation instructions

• All markdown renders cleanly and clearly

• No existing content was removed unnecessarily

[derek]

Thank you for your contribution. unfortunately, one or more of your commits are missing the required "Signed-off-by:" statement. Signing off is part of the Developer Certificate of Origin (DCO) which is used by this project.

Read the DCO and project contributing guide carefully, and amend your commits using the git CLI. Note that this does not require any cryptography, keys or special steps to be taken.

💡 Shall we fix this?

This will only take a few moments.

First, clone your fork and checkout this branch using the git CLI.

Next, set up your real name and email address:

git config --global user.name "Your Full Name"
git config --global user.email "you@domain.com"

Finally, run one of these commands to add the "Signed-off-by" line to your commits.

If you only have one commit so far then run: git commit --amend --signoff and then git push --force.
If you have multiple commits, watch this video.

Check that the message has been added properly by running "git log".