Document how to address review comments (#591)

docs/contributing/pull-request.md

@@ -2,11 +2,19 @@
 
 All changes in this repository should go through a pull request.
 
+## Review
+
+If a pull request has review comments that must be addressed by changing the code, those changes
+should be additional commits on top of the existing ones. Reviewers can then only review those
+additional changes.
+
+Pull requests are squash-merged such that only the title and description are used to build a single
+commit with all the changes.
+
 ## Documentation
 
-A pull request should document why it is needed. This could be either omitted (for typos and other
-obvious changes), a link to an issue, or a description in the pull request (for complex changes). In
-all cases, the pull request title should be clear and concise.
+If a pull request is not trivial (fixing a typo or other obvious change), it should have a
+description. In all cases, the pull request title should be clear and concise.
 
 If a pull request adds or modifies specified code, documentation should be added or updated to make
 sure everyone agrees on what the code should do.