docs(contributing): Add more information for contributors

Make it easy to understand how to contribute.

.github/CONTRIBUTING.md

@@ -9,7 +9,24 @@ I would recommend reading this guideline for a better development experience.
 
 <br />
 
+- [💬 Commit Message](#-commit-message)
 - [🔠 Branch Prefix](#-branch-prefix)
+- [❓ Pull Requests Title](#-pull-requests-title)
+- [🪵 Commit Log](#-commit-log)
+
+<br /><br />
+
+## 💬 Commit Message
+
+I recommend you to follow [Conventional Commits] with this format.
+
+```commit message
+type(scope): Description
+
+Body
+```
+
+[Conventional Commits]: https://www.conventionalcommits.org
 
 <br />
 
@@ -36,3 +53,18 @@ For the details of each label, please see [Labels](https://github.com/5ouma/opml
 |     Type: Meta      |               `^meta-`               |
 
 > Labels were generated with [@azu / github-label-setup](https://github.com/azu/github-label-setup)
+
+<br />
+
+## ❓ Pull Requests Title
+
+You don't need to add any prefixes like `feature` or `bug fix`
+to the Pull Requests title because I can recognize what kind of PR it is from labels.
+Please give a clear title.
+
+<br />
+
+## 🪵 Commit Log
+
+I do squash merge to the dev branch to keep the commit history clean.
+When merging your Pull Request, I'll add the Conventional Commits type and scope.