update readme for v3

[thboop]

No description provided.

[ericsciple]

Do we need a section to indicate whats new in v3?

[ericsciple]

Basically, what do users on v2 need to know?

[eregon]

I'd be useful to mention this node update is because of, quoting #689

Node 12 has an end of life on April 30, 2022.

I'm not sure why this should be a major release, it feels like a lot of churn for every workflow out there, most of them likely running on GH-hosted or recent-enough runners, and for which using a new 2.x release would have been better.

Does it mean - uses: actions/checkout@v2 which is used in so many workflows will no longer get updates/bug fixes?

This feels wrong, people using old runners should pin to a specific release or sha, people using reasonably recent runners should be able to get updates/bug fixes without needing to update all workflows with a new checkout major version every now and then (unless the semantics of the action does change and really needs a major release).

[jglick]

I think it would be worth explicitly mentioning here (and, more importantly, in https://github.com/actions/checkout/releases/tag/v3.0.0) that if you are using github.com (SaaS) you can safely accept the update and ignore the rest of the text.