contributing: Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contribution Guidelines
+
+We welcome contributions from the community! Whether you're fixing a bug, adding a feature, or improving documentation,
+your contributions help us improve this project for everyone!
+
+## Discussions
+
+If you're unsure about anything, or are not sure if a change should be made, feel free to ask questions by opening a
+discussion.
+
+## Before You Start
+
+* Before submitting a PR, verify that an issue exists that describes the bug fix or feature you want to contribute. If
+  there's no issue yet, please create one.
+* If you don't have access to the repo, fork to your on your GitHub user account, make code changes there, and then
+  create a PR against the `main` branch.
+* Add tests for any new features or bug fixes. Ideally, each PR increases the test coverage. We have a quality gate that
+  checks.
+* Follow the existing code style (follow [Effective Go](https://go.dev/doc/effective_go) where no obvious convention
+  exists).
+* Use the pull request template provided, and fill in all the details you can.
+* Document newly introduced methods and classes with godoc-style comments, and add inline comments to code that is not
+  self-documenting.
+* Separate unrelated changes into multiple PRs.
+* Commit messages should follow the conventions outlined at https://www.conventionalcommits.org/.
+
+## Writing Tests
+
+- Where possible, we try to write unit tests in a [table-driven](https://go.dev/wiki/TableDrivenTests) way (something
+  you’ll likely be familiar with if you’re familiar with Go).
+- This doesn’t mean a test _has_ to be written in this way but
+  if you find yourself in a situation where you are testing various inputs to a function that lead to different outputs, a
+  table-driven test will work well!
+
+
+
+## Creating the PR
+
+### PR Title
+
+Please follow the [Conventional Commits](https://www.conventionalcommits.org/) guidelines for PR titles (and commit
+titles).
+
+### PR Description
+
+Please split the PR description into the categories defined by https://keepachangelog.com, for example:
+
+```
+### Added
+- v1.1 Brazilian Portuguese translation.
+### Changed
+- Use frontmatter title & description in each language version template
+```
+
+If you are looking for help, or aren’t sure if something is an issue or not, please create a post under [Discussions](../discussions)
+instead.
+
+## Feedback
+
+Lastly, **don’t let review comments discourage you**! We appreciate any and all public contributions. Luno has an
+internal set of standards and guidelines we are committed to upholding on all of our repos, so we may request changes to
+bring any PRs in line with those.