Closes #1776

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

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: Documentation

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

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• (Not Applicable) All tests are passing: https://github.com/vuejs/vue-test-utils/blob/dev/.github/CONTRIBUTING.md#development-setup
• (Not Applicable) New/updated tests are included

Other information:

See #1776.

Changes:

• Make two sections, one for the Vue CLI installation and one for the manual installation.
• Clarify that code coverage is off by default.