docs(README.md): adding vue-skeleton-content-loader lib on the reference list

[willmendesneto]

Adding vue-skeleton-content-loader lib on the reference list for loader components to be used in Vue apps

General

✏️ Mark the necessary items without changing the structure of the PR template.

• Pull request template structure not broken

Type

ℹ️ What types of changes does your code introduce?

👉 Put an x in the boxes that apply

• Fix
• Feature

Checklist

ℹ️ Check all checkboxes - this will indicate that you have done everything in accordance with the rules in CONTRIBUTING.

👉 Put an x in the boxes that apply.

• Title as described
• Make sure you put things in the right category!
• Always add your items to the end of a list

Open Source

• Link description does not contain a link to an author / third-party resource
• The documentation (README) contains a description of the project, illustration of the project with a demo or screenshots and a CONTRIBUTING section
• The documentation is in English.
• The project is active and maintained.
• The project accepts contributions.
• Not a commercial product

Apps/Websites

• The website is available without errors or ssl certificate problems, and load in a reasonable amount of time.
• The website is using vuejs intensively. It should detect vue with vue-devtools.

  If you cannot detect vue with vue-devtools due to work at non public pages (e.g. for enterprise website), you can send Pull Request with screenshot that detected it.

• The website is original and not too simple. For that reason, blogs and simple landing pages are rejected.
• A commercial product using Vue, provided that guests could reasonably check out how Vue was used (i.e. A headless CMS which uses Vue for the Admin/editor Area and offers a free tier).