Add GitHub Action wrappers to README?

[anishathalye]

Just wanted to share these links in case you weren't aware of them:

• https://github.com/chabad360/htmlproofer
• https://github.com/anishathalye/proof-html

We used to use a CI setup where we did a gem install html-proofer and then called the htmlproofer CLI program, but we found it more convenient to switch to a setup that was a bit more integrated with GitHub actions and let us set parameters that weren't controllable through the CLI program, things like feeding in access tokens, using regex URL ignore patterns, and retrying when checking external links fails.

Sharing this in case you want to add links to the GitHub actions or add our CI setup to the "Real-life examples" section.

[gjtorikian]

Sure, I'll take a PR for this.