Add docs/contributing.md
Bug: dawn:23
Change-Id: I2cbf40c4079ea9d35c3d0c299c30e39b22eff80a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/48420
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
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+# How to contribute to Dawn
+
+First off, we'd love to get your contributions to Dawn!
+
+Everything helps other folks using Dawn and WebGPU: from small fixes and documentation
+improvements to larger features and optimizations.
+Please read on to learn about the contribution process.
+
+## One time setup
+
+### Contributor License Agreement
+
+Contributions to this project must be accompanied by a Contributor License
+Agreement. You (or your employer) retain the copyright to your contribution.
+This simply gives us permission to use and redistribute your contributions as
+part of the project. Head over to <https://cla.developers.google.com/> to see
+your current agreements on file or to sign a new one.
+
+You generally only need to submit a CLA once, so if you've already submitted one
+(even if it was for a different Google project), you probably don't need to do
+it again.
+
+### Gerrit setup
+
+Dawn's contributions are submitted and reviewed on [Dawn's Gerrit](https://dawn-review.googlesource.com).
+
+Gerrit works a bit differently than Github (if that's what you're used to):
+there are no forks. Instead everyone works on the same repository. Gerrit has
+magic branches for various purpose:
+
+ - `refs/for/<branch>` (most commonly `refs/for/main`) is a branch that anyone
+can push to that will create or update code reviews (called CLs for ChangeList)
+for the commits pushed.
+ - `refs/changes/00/<change number>/<patchset>` is a branch that corresponds to
+the commits that were pushed for codereview for "change number" at a certain
+"patchset" (a new patchset is created each time you push to a CL).
+
+#### Gerrit's .gitcookies
+
+To push commits to Gerrit your `git` command needs to be authenticated. This is
+done with `.gitcookies` that will make `git` send authentication information
+when connecting to the remote. To get the `.gitcookies`, log-in to [Dawn's Gerrit](https://dawn-review.googlesource.com)
+and browse to the [new-password](https://dawn.googlesource.com/new-password)
+page that will give you shell/cmd commands to run to update `.gitcookie`.
+
+#### Set up the commit-msg hook
+
+Gerrit associates commits to CLs based on a `Change-Id:` tag in the commit
+message. Each push with commits with a `Change-Id:` will update the
+corresponding CL.
+
+To add the `commit-msg` hook that will automatically add a `Change-Id:` to your
+commit messages, run the following command:
+
+```
+f=`git rev-parse --git-dir`/hooks/commit-msg ; mkdir -p $(dirname $f) ; curl -Lo $f https://gerrit-review.googlesource.com/tools/hooks/commit-msg ; chmod +x $f
+```
+
+Gerrit helpfully reminds you of that command if you forgot to set up the hook
+before pushing commits.
+
+## The code review process
+
+All submissions, including submissions by project members, require review.
+
+### Discuss the change if needed
+
+Some changes are inherently risky, because they have long-term or architectural
+consequences, contain a lot of unknowns or other reasons. When that's the case
+it is better to discuss it on the [Dawn Matrix Channel](https://matrix.to/#/#webgpu-dawn:matrix.org)
+or the [Dawn mailing-list](https://groups.google.com/g/dawn-graphics/members).
+
+### Pushing changes to code review
+
+Before pushing changes to code review, it is better to run `git cl presubmit`
+that will check the formatting of files and other small things.
+
+Pushing commits is done with `git push origin HEAD:refs/for/main`. Which means
+push to `origin` (i.e. Gerrit) the currently checkout out commit to the
+`refs/for/main` magic branch that creates or updates CLs.
+
+When code review asks for changes in the commits, you can amend them any way
+you want (small fixup commit and `git rebase -i` are crowd favorites) and run
+the same `git push origin HEAD:refs/for/main` command.
+
+### Tracking issues
+
+We usually like to have commits associated with issues in [Dawn's issue tracker](https://bugs.chromium.org/p/dawn/issues/list)
+so that commits for the issue can all be found on the same page. This is done
+by adding a `Bug: dawn:<issue number>` tag in the commit message. It is also
+possible to reference Chromium or Tint issues with `Bug: tint:<issue number>` or
+`Bug: chromium:<issue number>`.
+
+Some small fixes (like typo fixes, or some one-off maintenance) don't need a
+tracking issue. When that's the case, it's good practice to call it out by
+adding a `Bug: None` tag.
+
+It is possible to make issues fixed automatically when the CL is merged by
+adding a `Fixed: <project>:<issue number>` tag in the commit message.
+
+### Iterating on code review
+
+Dawn follows the general [Google code review guidelines](https://google.github.io/eng-practices/review/).
+Most Dawn changes need reviews from two Dawn committers. Reviewers will set the
+"Code Review" CR+1 or CR+2 label once the change looks good to them (although
+it could still have comments that need to be addressed first). When addressing
+comments, please mark them as "Done" if you just address them, or start a
+discussion until they are resolved.
+
+Once you are granted rights (you can ask on your first contribution), you can
+add the "Commit Queue" CQ+1 label to run the automated tests for Dawn. Once the
+CL has CR+2 you can then add the CQ+2 label to run the automated tests and
+submit the commit if they pass.
+
+The "Auto Submit" AS+1 label can be used to make Gerrit automatically set the
+CQ+2 label once the CR+2 label is added.
