[SVLS-10001] docs: add CONTRIBUTING.md with commit signing steps

CONTRIBUTING.md

@@ -0,0 +1,40 @@
+# Contributing
+
+Thank you for your interest in the Datadog Lambda Extension. This page tells you what a pull request needs before we can merge it.
+
+## Sign your commits
+
+GitHub blocks the merge of a pull request until every commit in it has a verified signature. Set up signing before you push your first commit:
+
+1. Create a GPG or SSH signing key, and add it to your GitHub account as a signing key. See [Signing commits](https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits) in the GitHub documentation.
+2. Configure Git to sign every commit. For an SSH key:
+
+   ```sh
+   git config --global gpg.format ssh
+   git config --global user.signingkey ~/.ssh/id_ed25519.pub
+   git config --global commit.gpgsign true
+   ```
+
+3. Commit with an email address that you verified on your GitHub account. GitHub shows a commit as verified only when the signing key and the committer email belong to the same account.
+
+If you already pushed commits without a signature, sign them again and force-push. Replace `upstream` with the name of your remote for `DataDog/datadog-lambda-extension`:
+
+```sh
+git rebase --exec 'git commit --amend --no-edit -n -S' $(git merge-base HEAD upstream/main)
+git push --force-with-lease
+```
+
+## Open the pull request
+
+- Fill in the pull request template.
+- Keep **Allow edits by maintainers** selected. Then a maintainer can push small changes to your branch, for example to sign the commits again.
+
+## Run the checks
+
+The Rust code is in `bottlecap/`. Before you push, run these commands from that directory. CI runs the same checks:
+
+```sh
+cargo fmt --all -- --check
+cargo clippy --workspace --all-targets --features default
+cargo nextest run --workspace
+```

README.md

@@ -97,7 +97,7 @@ You can also open an issue for a feature request.
 
 ## Contributing
 
-If you find an issue with this package and have a fix, please feel free to open a pull request following the [procedures](https://github.com/DataDog/datadog-agent/blob/main/docs/public/guidelines/contributing.md).
+If you find an issue with this package and have a fix, please feel free to open a pull request following the [procedures](https://github.com/DataDog/datadog-agent/blob/main/docs/public/guidelines/contributing.md). See [CONTRIBUTING.md](CONTRIBUTING.md) for what a pull request in this repository needs, such as signed commits.
 
 ## Community