Add to CONTRIBUTING.md

CONTRIBUTING.md

@@ -9,8 +9,15 @@ All submissions, including submissions by project members, require review. We
 use GitHub pull requests for this purpose. Consult [GitHub Help] for more
 information on using pull requests.
 
+Every pull request should document all changes made in the [changelog].
+- The format is always `- [what changed]. ([#pr-number] by [@username])`.
+- The number can be guessed or added after creating the pull request.
+- Links to all pull requests are recorded at the bottom of the unreleased section.
+- If you are a new contributor, please add your name to the others at the bottom of the CHANGELOG.md.
+
 If your name does not already appear in the [AUTHORS] file, please feel free to
 add it as part of your patch.
 
 [GitHub Help]: https://help.github.com/articles/about-pull-requests/
 [AUTHORS]: AUTHORS
+[changelog]: CHANGELOG.md