Expand `corepack use` documentation description

更新 README 的 corepack use 部分，以链接的 issue 607 测试示例及其 shell 命令作为行为参考。记录该命令如何通过目录层级查找 package.json，以及在不存在文件或 packageManager 字段时会创建什么；当这两种行为都得到清晰描述时，即视为完成。

Issue

Under README > corepack use the complete description currently says:

corepack use <name[@]>

When run, this command will retrieve the latest release matching the provided descriptor, assign it to the project's package.json file, and automatically perform an install.

package.json location

There is no description of how the command navigates the directory hierarchy to locate a possible existing package.json to modify.

Using the test example of @arcanis in https://github.com/nodejs/corepack/issues/607#issuecomment-2609834263 shows that it operates on the root of a hierarchy:

corepack enable yarn
cd $(mktemp -d)
yarn init -2 -w
mkdir -p packages/foo
echo '{}' > packages/foo/package.json
jq .packageManager package.json
(cd packages/foo && corepack use yarn@4.5.0)
jq .packageManager package.json

In this example case, the result is that:

• packages/foo/package.json contains {}

• package.json contains

  {
    "name": "tmp.IcgYOXK7aS",
    "packageManager": "yarn@4.5.0+sha512.837566d24eec14ec0f5f1411adb544e892b3454255e61fdef8fd05f3429480102806bac7446bc9daff3896b01ae4b62d00096c7e989f1596f2af10b927532f39",
    "workspaces": [
      "packages/*"
    ]
  }
  

package.json creation

If no package.json file is found, then one is created. This effect of the command is not described.

Suggestion

For the README > corepack use section:

• Add text to explain how the location for the package.json file is determined.
• Add text which describes how a package.json file and / or a packageManager field is created if they don't exist.

Related

• https://github.com/nodejs/corepack/issues/607