Update README.md to provide installation guide for mac / linux users

[Owen-CH-Leung]

• 📙 Documentation : Update the README.md to provide a detailed installation guide for mac & linux users.

Closes #320 .

📜 Checklist

• [X] Works on the ./demo sub directory
• [X] Test coverage is excellent and passes
• [X] Documentation is thorough

[Owen-CH-Leung]

@drahnr Just fixed the failing CI test. Can I seek your review ? Many thanks

[drahnr]

Could you squash it into one commit, then we can merge it :) Thank you!

[Owen-CH-Leung]

A few nits, otherwise LGTM. Thank you! Sorry for the long delay, life happened

Thanks. Updated README.md as per your suggestions =)

[Owen-CH-Leung]

bad rebase =( Let me create another branch instead

[Owen-CH-Leung]

@drahnr Here:

https://github.com/drahnr/cargo-spellcheck/pull/331