📝 Docs: Indicate which commands install package managers automatically

[JoshuaKGoldberg]

Coming over from eslint/rfcs#127 (comment): it wasn't clear to me from reading README.md whether ni, nci, or other commands will automatically install pnpm, Yarn, etc. if the user's system doesn't have them already. Can this be added to the documentation?

The closest equivalent docs I could find were:

• Enhancement for non-interactive use cases #178 (comment): that nci does it automatically via autoinstall
• Add support for autoinstall in nr command #247: asking to add autoinstall to nr

...but the autoinstall word doesn't exist in README.md right now.