Update README.md with input/output kebab-case naming convention

[teemingc]

This PR updates the README.md which documents the input/output names. According to the release notes from https://github.com/changesets/action/releases/tag/v2.0.0-next.3 they're supposed to use kebab-case rather than camel case now.

Also adds some new inputs which aren't documented such as push-git-tags and github-token

[changeset-bot]

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Latest commit: e47e679

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