GitHub Action for Prettier + Add link to actionlint action

[EPMatt]

Hi there,

first of all I really want to spend some words to thank all the reviewdog contributors for maintaining such a useful code review tool to the dev community. Keep going! 🚀

While looking at the available GitHub Actions for reviewdog I noticed no one was available for Prettier. Given the popularity and utility of such a widely used code formatter, I decided to build one and here we are. :)

The action is available here. If you have any suggestion or improvement, I'd be happy to hear your opinion. :)

This PR just adds a link to the action's repository to the list of public reviewdog GitHub Actions, provided in the README file. Moreover, it also adds a link to the reviewdog/action-actionlint action which I noticed was missing in the README.

Thank you!

• Updated Unreleased section in CHANGELOG or it's not notable changes.

[mgrachev]

@EPMatt 👋 Thank you for your contribution 👍