Add manners for reducing reviewers' workload to `CONTRIBUTING.md`

[junkmd]

We have been promoting measures to increase the number of contributors. Contributors will be PR reviewees. As the number of contributors increases, so does the reviewer's workload.

We need to encourage contributors to submit PRs that are less burdensome for reviewers and more likely to be approved.

For example, Small CLs in Google Engineering Practices Documentation shows great practices about these.

translated doc

• "Small CLs" in ja-jp
• "Google Engineering Practices Documentation" in ja-jp

Let’s add them to CONTRIBUTING.md