Update README.md

[bruce-usab]

Delete 2016 references. Add link to WAI GitHub Onboarding wiki.

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[kfranqueiro]

One suggestion from the WAI Team:

https://www.w3.org/guide/github/ also exists, and we wonder if that may make sense to either:

• Link to the W3C on GitHub guide within the WCAG WG wiki page you're linking to? (Maybe it can replace some of the links there?)
• Possibly replace the link this PR adds to the README outright? (But I suspect the wiki page might cover more AGWG specifics than the W3C on GitHub guide does)

[patrickhlauke]

• Link to the W3C on GitHub guide within the WCAG WG wiki page you're linking to? (Maybe it can replace some of the links there?)

that would make most sense to me