Adds simple CONTRIBUTING.md (#17)

This adds an initial CONTRIBUTING.md with some comments
about DCO as well as a generic PR review processes.

Signed-off-by: Takeshi Yoneda <[email protected]>

CONTRIBUTING.md

@@ -0,0 +1,42 @@
+# Contributing
+
+We welcome contributions from the community. Please read the following guidelines carefully to maximize the chances of your PR being merged.
+
+## Coding Style
+
+To ensure your change passes format checks, run `make precommit` and ensure all checks pass.
+
+## DCO
+
+We require DCO signoff line in every commit to this repo.
+
+The sign-off is a simple line at the end of the explanation for the
+patch, which certifies that you wrote it or otherwise have the right to
+pass it on as an open-source patch. The rules are pretty simple: if you
+can certify the statement in [developercertificate.org](https://developercertificate.org/)
+then you just add a line to every git commit message:
+
+    Signed-off-by: Joe Smith <[email protected]>
+
+using your real name (sorry, no pseudonyms or anonymous contributions.)
+
+You can add the sign off when creating the git commit via `git commit -s`.
+
+## Code Reviews
+
+* The pull request title should describe what the change does and not embed issue numbers.
+The pull request should only be blank when the change is minor. Any feature should include
+a description of the change and what motivated it. If the change or design changes through
+review, please keep the title and description updated accordingly.
+* **A single approval is sufficient to merge**. If a reviewer asks for
+changes in a PR they should be addressed before the PR is merged,
+even if another reviewer has already approved the PR.
+* During the review, address the comments and commit the changes
+**without squashing the commits**. This facilitates incremental reviews
+since the reviewer does not go through all the code again to find out
+what has changed since the last review. When a change goes out of sync with main,
+please rebase and force push, keeping the original commits where practical.
+* Commits are squashed prior to merging a pull request, using the title and PR description
+as commit message by default. Maintainers may request contributors to
+edit the pull request title and description to ensure that it remains descriptive as a
+commit message. Alternatively, maintainers may change the commit message directly at the time of merge.