docs: Split out commit guidelines into their own section

The commit guidelines feel like they're relevant before documentation
about the code review process - they could conceivably apply while
developing a change.

These often seem to get missed and raised during reviews - giving them
their own section may increase visibility.

docs/contributing.md

@@ -18,12 +18,7 @@ You generally only need to submit a CLA once, so if you've already submitted one
 (even if it was for a different project), you probably don't need to do it
 again.
 
-### Code reviews
-
-All submissions, including submissions by project members, require review. We
-use GitHub pull requests for this purpose. Consult
-[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
-information on using pull requests.
+### Commit guidelines
 
 Unlike many GitHub projects (but like many VCS projects), we care more about the
 contents of commits than about the contents of PRs. We review each commit
@@ -51,6 +46,13 @@ guide if you're new to writing good commit messages. We are not particularly
 strict about the style, but please do explain the reason for the change unless
 it's obvious.
 
+### Code reviews
+
+All submissions, including submissions by project members, require review. We
+use GitHub pull requests for this purpose. Consult
+[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
+information on using pull requests.
+
 When you address comments on a PR, don't make the changes in a commit on top (as
 is typical on GitHub). Instead, please make the changes in the appropriate
 commit. You can do that by creating a new commit on top of the initial commit