Stabilize Node.js to 16.20.0 for all uses and developer docs

[code-ape]

Issues

none

Changes

1. Stabilized Node.js to latest 16 LTS release for development and CI, via .nvmrc and .github/workflows/test.yml.
2. Added local NPM registry config to honor existing Yarn lock file, via .npmrc.
3. Added developer docs to make it easier for others to use this and contribute to this project, via README.md.

[code-ape]

Just checking if there's something I should do to get this reviewed? (+CC @dfreeman)

[dfreeman]

Hey @code-ape, thank you for the PR and apologies for the wait! I'll aim to carve out time this week to look through the pending PRs and get things sorted.

We aren't actually actively using this action ourselves at Salsify any more, so the repo doesn't get eyes as frequently as would be ideal. Given that, if you'd be interested in taking ownership of it, I'd be happy to look into either transferring the repo to you or archiving this one and pointing to one you own in the README here.