docs: add macOS workaround for killed install command

[sshevlyagin]

Summary

• Some macOS users (myself included) see the one-liner installer get killed by the shell:

  $ bash <(curl -sL https://raw.githubusercontent.com/databricks-solutions/ai-dev-kit/main/install.sh)
  zsh: killed     bash
  

• Adds a short README note under the Mac / Linux basic install pointing users to a download-then-execute fallback that avoids process substitution.

Test plan

• Render README on GitHub and confirm the note appears under the basic install snippet.
• Verify the fallback commands work end-to-end on macOS.

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