461 add review pr guide [skip ci] (#462)

Fixes #461 .

### Description
This PR is used to enhance the contributing guide. A section called
`Reviewing pull requests` is added.
This PR refers to:

https://github.com/Project-MONAI/MONAI/blob/60620e46645d131f3fe05e98f4c91e1fbda9bf13/CONTRIBUTING.md?plain=1#L328

### Status
**Ready**

### Please ensure all the checkboxes:
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Codeformat tests passed locally by running `./runtests.sh
--codeformat`.
- [x] In-line docstrings updated.

Signed-off-by: Yiheng Wang <[email protected]>

CONTRIBUTING.md

@@ -193,9 +193,21 @@ Ideally, the new branch should be based on the latest `dev` branch.
 1. Reviewer and contributor may have discussions back and forth until all comments addressed.
 1. Wait for the pull request to be merged.
 
+## Reviewing pull requests
+
+All code review comments should be specific, constructive, and actionable.
+1. Check [the CI/CD status of the pull request][github ci], make sure all CI/CD tests passed before reviewing (contact the branch owner if needed).
+1. Read carefully the descriptions of the pull request and the files changed, write comments if needed.
+1. Make in-line comments to specific code segments, [request for changes](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-request-reviews) if needed.
+1. Review any further code changes until all comments addressed by the contributors.
+1. Comment to trigger `/black` for optional auto code formatting.
+1. [Maintainers] Review the changes and comment `/build` to trigger internal full tests.
+1. Merge the pull request to the dev branch.
+1. Close the corresponding task ticket on [the issue list][monai model zoo issue list].
+
+[github ci]: https://github.com/Project-MONAI/model-zoo/actions
 [monai model zoo issue list]: https://github.com/Project-MONAI/model-zoo/issues
 
-
 ## Validate and release
 
 As for a pull request, a CI program will try to download all large files if mentioned and do several validations. If the pull request is approved and merged, the full bundle (with all large files if exists) will be archived and send to [Releases](https://github.com/Project-MONAI/model-zoo/releases).