Update "What's New" of v2 in README page

[shenxianpeng]

With many features have been introduced in v2, and #87 can not cover all the features in v2, we may need to update the "What's New" section.

[2bndy5]

Which features are you thinking about?

We could generate a CHANGELOG.md file when the docs build is run. But the changes shown in cpp-linter-action releases are not very descriptive about changes in the cpp-linter package.

[shenxianpeng]

I assume features like no-lgtm, step-summary, tidy-review, format-review, jobs, etc in Inputs and Outputs page when the version starts with 2.x.x

[2bndy5]

oh ok. Sounds good!

[github-actions]

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