Add support GitHub Actions to README.

[M-Yamashita01]

This PR is based on https://github.com/codecov/codecov-ruby/pull/78

The https://github.com/codecov/codecov-ruby/pull/78 PR did not have GitHub Actions support written in README. Therefore, we added GitHub Actions support.

[height[bot]]

Link Height tasks by mentioning a task ID in the pull request title or commit messages, or description and comments with the keyword link (e.g. "Link T-123").

💡Tip: You can also use "Close T-X" to automatically close a task when the pull request is merged.

[thomasrockhu-codecov]

Closing due to deprecation