Document usage with pnpm

To get XO to work on a pnpm directory, all ESLint-related XO-dependencies must be installed as a devDependency. This can be accomplished with the following command:

(
  export PKG=xo;
  npm info "xo@latest" dependencies --json | command sed 's/[\{\},]//g ; s/: /@/g' | grep "\"eslint" | xargs pnpm i -D "$PKG@latest"
)

To fix the eslint-plugin-import issues (#270), eslint-import-resolver-node must also be installed:

pnpm install --save-dev eslint-import-resolver-node

Finally, if "prettier" is set to true in XO’s config, it must be installed too:

pnpm install --save-dev prettier

I think it would be prudent to document this in the README.

[jorgegonzalez]

pnpm is not a “random” package manager. It’s used by several major companies and has a dedicated and growing user base.

And the idea is that if a user has chosen pnpm as their primary package manager, and they want to use XO, they can. If a user wants to run XO on a project or environment where pnpm is required, they can. Running 3 commands isn’t that much trouble, and properly documenting helps everyone.

[jorgegonzalez]

Closing this, as someone searching for “xo and pnpm” will be able to find this issue easily. Didn’t mean to annoy or upset anyone.