lint-staged no longer requires the "git add" command

[JoseGoncalves]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:
Since v10 lint-staged does not require to add the git add command.
Additional info: lint-staged/lint-staged#775

[sodatea]

The current lint-staged used by Vue CLI is still at 9.x. So I'm afraid we'll have to keep the documentation as-is until we've got the time to update the dependencies.

[JoseGoncalves]

If that is the case, I think the documentation should be updated indicating that the user shoud install v9 of lint-staged, i.e., add some note to execute in his project:

$ npm i -D lint-staged@9

otherwise, a user will install the latest version (v10), and will receive a warning on every commit of .vue or .js files:

 > running pre-commit hook: lint-staged
⚠ Some of your tasks use `git add` command. Please remove it from the config since all modifications made by tasks will be automatically added to the git commit index.

[sodatea]

Fixed in #6136