contrib guide: Describe how to resolve merge conflicts

contributors/guide/github-workflow.md

@@ -93,7 +93,9 @@ git rebase upstream/master
 Please don't use `git pull` instead of the above `fetch` and
 `rebase`. Since `git pull` executes a merge, it creates merge commits. These make the commit history messy
 and violate the principle that commits ought to be individually understandable
-and useful (see below). 
+and useful (see below). However, once your PR has been created, merge commits
+become preferable to rebasing to [help your
+reviewers](./pull-requests.md#resolving-merge-conflicts).
 
 You might also consider changing your `.git/config` file via
 `git config branch.autoSetupRebase always` to change the behavior of `git pull`, or another non-merge option such as `git pull --rebase`.

contributors/guide/pull-requests.md

@@ -31,6 +31,7 @@ It should serve as a reference for all contributors, and be useful especially to
   - [Comments Matter](#comments-matter)
   - [Test](#test)
   - [Squashing](#squashing)
+  - [Resolving Merge Conflicts](#resolving-merge-conflicts)
   - [Commit Message Guidelines](#commit-message-guidelines)
   - [It's OK to Push Back](#its-ok-to-push-back)
   - [Common Sense and Courtesy](#common-sense-and-courtesy)
@@ -368,6 +369,32 @@ if this label is used, please know the following:
   extra effort. It can also speed up merging because squashing by hand implies getting another LGTM
   from a reviewer and re-run of the CI tests.
 
+## Resolving Merge Conflicts
+
+The Kubernetes project maintains high development velocity, so it's likely other
+PRs will merge which introduce merge conflicts while Feature-X is being
+reviewed. Both the GitHub UI and `@k8s-ci-robot` will point out when a PR
+contains merge conflicts that need to be resolved with the upstream branch.
+
+Like when [addressing other feedback](#squashing), maintaining the
+history of the changes that have already been reviewed is crucial for enabling
+reviewers to determine exactly what's changed between updates. To resolve merge
+conflicts this way, use `git merge` instead of `git rebase`.
+
+A new merge commit shows your reviewer exactly where the conflicts occurred and
+how they were resolved. Rebasing _rewrites every commit on the branch_ and the
+old commits are _lost entirely_. Comparing the previous tip of the branch with
+the rebased tip, a one-line merge conflict resolution may be buried in a diff
+spanning hundreds of files and tens of thousands of lines! Savvy reviewers may
+know some tricks that help reviewing rebased branches (GitHub's force-push diffs
+and viewed file tracking, `git range-diff`), but those aren't sufficient in all
+cases to isolate the new changes introduced by your PR from upstream changes
+made by other PRs. _Merge! Don't rebase!_
+
+`@k8s-ci-robot` will add the `do-not-merge/contains-merge-commits` label to your
+PR. This looks scary, but it is okay! It's only a reminder to rebase and squash
+when your PR has been reviewed and is ready to merge.
+
 ## Commit Message Guidelines
 
 PR comments are not represented in the commit history.