Discuss and document the Pull Request review process

During the release v0.1.0 to v0.2.1, @weiji14 and I were the only active maintainers of the project. We usually submitted PRs, waited for approval from the other one, and then merged PRs immediately after approval.

Now the team is growing quickly, and we have more maintainers and contributors commenting on PRs, so we usually do **NOT** merge PRs immediately after **one** approval. The review process helps make the project better, but also becomes longer. I'm not sure if we have reached a consensus about _when_ a PR should be merged and _who_ should push the merge button (currently most PRs were merged by @weiji14 and me). New contributors may also be confused why their PRs are already approved but still not merged.

So I feel we should discuss the PR review process and document it in the contributing/maintenance guides.

Here is the PR review process I think we're loosely following:

- Open an issue and discuss it
- Submit a **draft** PR and work on it
- Add labels and milestones
- Several rounds of review and revision
- Mark the PR as Ready for Review
- Formal review process
- Someone approves the PR, and adds the "final review call" label
- Merge the PR if approved by _X_ reviewers or no further comments after _X_ hours

Thoughts and comments on the review process?