Improve contributing docs

[frontendlane]

First time contributor checklist:

• I have read the README and Contributor Guidelines
• I have signed the Contributor Licence Agreement

Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A pnpm run ready run passes successfully (more about tests here)
• My changes are ready to be shipped to users

Description

I wanted to contribute to this project but hit a brick wall because I'm running an old version of macOS. So here's a couple of documentaion improvements in CONTRIBUTING.md:

1. use a one-liner to install and use the correct node version
2. add a heads-up about minimum macos version to build the project