CONTRIBUTING.md: Fill out section on pull requests.

CONTRIBUTING.md

@@ -121,11 +121,49 @@ TODO
         into fixing and (2) otherwise isn't being prioritized are likely to
         be closed.
 
-### What to expect when you submit a PR
-
-TODO: It is strongly recommended that you validate your contributions before
-you make significant efforts…
-
-The "Assigned" field on a PR indicates who has taken responsibility
-for reviewing the PR, not who is responsible for the content of the
-PR.
+### Pull requests
+
+The "Assigned" field on a pull request indicates who has taken
+responsibility for shepherding it through the review process, not who
+is responsible for authoring it. The assignee is usually the reviewer,
+but they can also delegate the review to someone else. The intent of
+assignment is simply to ensure that pull requests don't get neglected.
+
+A draft pull request is taken to be not yet ready for review. Marking
+drafts as such helps the maintainers triage review work.
+
+When one pull request builds on another, please put "Depends on #NNNN"
+towards the top of its description. This helps maintainers notice that
+they shouldn't merge it until its ancestor has been approved. Don't
+use the "draft pull request" status to indicate dependency.
+
+If a pull request contains multiple commits, please indicate whether
+they need to be squashed into a single commit before they're merged,
+or if they're ready to rebase onto `trunk` as they stand. In the
+latter case, please ensure that each commit passes all CI tests, so
+that we can continue to bisect along `trunk` to isolate bugs.
+
+Pull requests should not contain merge commits. Please rebase on
+`trunk` before submitting a pull request. (Draft pull requests may
+contain anything you like.)
+
+#### Large pull requests are not accepted
+
+Large projects must be broken into series of smaller pull requests
+that can be reviewed and discussed independently. WGPU's experience
+with large, complex pull requests has been bad enough that we now
+simply reject them, without trying to assess their merits.
+
+- Large pull requests are very difficult to review effectively. It is
+  common for us to debug a problem in WGPU and find that it was
+  introduced by some massive pull request that we had accepted,
+  showing that we obviously hadn't understood it as well as we'd
+  thought.
+
+- A giant pull request obviously represents a significant effort on
+  the part of the author. At a personal level, it is quite stressful
+  to question its design decisions, knowing that changing them will
+  require the author to essentially reimplement the project from
+  scratch. Giant pull requests effectively arrogate the ability to
+  make design decisions from the maintainers, whereas incremental
+  changes can be discussed and revised without drama.