Add contributing guide and code of conduct documents

CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+Please refer to the [Couchbase Community Guidelines](https://www.couchbase.com/community-guidelines)
+which applies to community members as well as employees.

CONTRIBUTING.md

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+# Contributing
+
+Welcome, and thanks for lending a hand! Before we get started, please read the
+[Couchbase Code of Conduct](CODE_OF_CONDUCT.md).
+
+## Discussion
+
+We'd love for you to join us on the [Couchbase Forum](https://couchbase.com/forums).
+For more interactive discussions we have `#sdks` channel on our [Discord](https://discord.com/invite/sQ5qbPZuTh).
+
+## Reporting Issues
+
+The simplest way to help is to report bugs or request new features.
+
+We use Jira for issue tracking: [https://issues.couchbase.com/projects/PCBC/](https://issues.couchbase.com/projects/PCBC/)
+
+To create an issue, or comment on an existing issue, you'll need to sign up
+for an account. It's quick and painless (or if it's not, click on the
+"Report a problem" link on the signup page or get in touch via the forum).
+
+## Pull Requests
+
+We use Github for code review. But to allow us to integrate your changes, you need to register for a Gerrit account and
+sign a Contributor License Agreement (CLA).
+
+> We acknowledge Gerrit can be intimidating for new users.
+> We're considering alternates that will make it easier for GitHub
+> natives to contribute.
+
+  1. Visit [https://review.couchbase.org](https://review.couchbase.org) and register for an account.
+  2. Review our [Contributor Licence Agreement (CLA)](https://review.couchbase.org/static/individual_agreement.html).
+  3. Agree to the CLA by visiting [https://review.couchbase.org/settings/#Agreements](https://review.couchbase.org/settings/#Agreements)
+     Otherwise, we won't be able to merge your changes.
+
+If you need any help along the way, please contact us on the [Couchbase Forum](https://couchbase.com/forums).
+
+## Testing Your Changes
+
+This repository is configured to use [Github Actions CI](https://docs.github.com/en/actions).
+You can run test suite in your fork before submitting changes. The pipeline run
+tests on variety of platforms that we support and also produces binary builds as
+artifacts that could be downloaded for local testing. The definition of
+pipelines could be found here:
+[.github/workflows](https://github.com/couchbase/couchbase-php-client/tree/main/.github/workflows)