Documentation: add instructions to configure package.json with "packageManager": "xyz"

[NiloCK]

Under Usage, several mentions are made to corepack checking whether the project is configured for a specific package manager, but no instructions are given for configuring a project for a specific package manager.

The yvm deprecation notice and its linked example were the first instruction I found.

[jakobbouchard]

It's especially annoying when the project uses pnpm, but hasn't specified it in package.json and corepack assumes the project uses yarn instead, if yarn is also enabled.

[merceyz]

It's especially annoying when the project uses pnpm, but hasn't specified it in package.json and corepack assumes the project uses yarn instead, if yarn is also enabled.

Corepack doesn't assume anything like that, if you're seeing otherwise please open a bug report.