docs: Add section on crediting the work of others.

docs/contributing/continuing-unfinished-work.md

@@ -12,6 +12,7 @@ are the steps required:
 1. [Find work to be completed.](#find-work-to-be-completed)
 1. [Review existing work and feedback.](#review-existing-work-and-feedback)
 1. [Decide how to use prior work.](#decide-how-to-use-prior-work)
+1. [Credit prior work in your commit history.](#credit-prior-work-in-your-commit-history)
 1. [Present your pull request.](#present-your-pull-request)
 
 ## Find work to be completed
@@ -47,6 +48,7 @@ If prior work looks like a good start:
 1. Carefully address any open feedback from reviewers.
 1. Make any other changes you think are needed, including completing any parts
    of the work that had not been finished.
+1. Make sure the work of others is [properly credited](#credit-prior-work-in-your-commit-history).
 1. [Self-review](../contributing/code-reviewing.md), test, and revise the work,
    including potentially [splitting out](../contributing/commit-discipline.md)
    preparatory commits to make it easier to read. You should be proud of the
@@ -59,6 +61,21 @@ Otherwise, you can:
 1. Go through reviewer feedback on prior work. Would any of it apply to the
    changes you're proposing? Be sure to address it if so.
 
+## Credit prior work in your commit history
+
+When you use or build upon someone else's unmerged work, it is both professionally
+and ethically necessary to [properly credit][coauthor-git-guide] their contributions
+in the commit history of work that you submit.
+
+The most direct way to credit someone else's work is with a `Co-authored-by:` line
+after a blank line at the end of your commit message:
+
+    Co-authored-by: Greg Price <greg@zulip.com>
+
+You may find it necessary to make use of tools such as `git commit --amend -C` or
+`git commit --amend --reset-author` in instances where you squash commits in such a
+way that preserves the wrong authorship information.
+
 ## Present your pull request
 
 In addition to the usual [guidance](../contributing/reviewable-prs.md) for
@@ -73,12 +90,9 @@ putting together your pull request, there are a few key points to keep in mind.
   - Test the work carefully, even if others have tested it before. There may be
     problems that the reviewers missed, or that were introduced by rebasing across other changes.
 
-- **Give credit where credit is due.** In the commit message for any commits
+- **Give credit where credit is due.** Reviewers should be able to examine your
-  that use somebody else's work, [credit][coauthor-git-guide] co-authors by
+  commit history and see that you have [properly credited](#credit-prior-work-in-your-commit-history)
-  adding a `Co-authored-by:` line after a blank line at the end of your commit
+  the work of others.
-  message:
-
-      Co-authored-by: Greg Price <greg@zulip.com>
 
 - **Explain the relationship between your PR and prior work** in the description
   for your pull request.