Add style guidelines for commit messages (#3752)

Author: JukkaL

CONTRIBUTING.md

@@ -112,6 +112,22 @@ Core developers should follow these rules when processing pull requests:
 * Use "[Squash and merge](https://github.com/blog/2141-squash-your-commits)"
   to merge PRs.
 * Delete branches for merged PRs (by core devs pushing to the main repo).
+* Edit the final commit message before merging to conform to the following
+  style (we wish to have a clean `git log` output):
+  * When merging a multi-commit PR make sure that the commit message doesn't
+    contain the local history from the committer and the review history from
+    the PR. Edit the message to only describe the end state of the PR.
+  * Make sure there is a *single* newline at the end of the commit message.
+    This way there is a single empty line between commits in `git log`
+    output.
+  * Split lines as needed so that the maximum line length of the commit
+    message is under 80 characters, including the subject line.
+  * Capitalize the subject and each paragraph.
+  * Make sure that the subject of the commit message has no trailing dot.
+  * Use the imperative mood in the subject line (e.g. "Fix typo in README").
+  * If the PR fixes an issue, make sure something like "Fixes #xxx." occurs
+    in the body of the message (not in the subject).
+  * Use Markdown for formatting.
 
 
 Issue-tracker conventions