docs: add contributing guide

CONTRIBUTING.md

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+# Contributing Guidelines
+
+## Code of Conduct
+This project and everyone participating in it is governed by our
+[Code of Conduct](./CODE_OF_CONDUCT.md). By participating, you are expected to
+uphold this code. Please report unacceptable behavior to
+[email protected].
+
+## How can I contribute?
+- Reporting bugs
+- Proposing new features or improvements
+  - For bug reports & proposals, consider the following:
+  - Always be respectful, and mind the [Code of Conduct](./CODE_OF_CONDUCT.md)
+  - Check if someone else already reported that bug or proposed that idea.
+  - Try to be thorough and detailed with your explanations, to help others to
+    understand them and take proper action.
+- Improving the current documentation
+- Contributing code
+  - Always be respectful, and mind the [Code of Conduct](./CODE_OF_CONDUCT.md)
+  - Backwards compatibility is almost sacred, please try to preserve it.
+  - Try to respect the current coding style, to avoid style inconsistencies.
+
+## Code Contributions: Acceptance Criteria
+
+In order for us to accept contributions, the merge request must fulfill certain
+requirements:
+
+### Style Guide
+
+There is no "official" style guide, although we enforce style through automated
+tools, such as [Prettier](https://prettier.io/) and
+[ESLint](https://eslint.org/).
+
+### Commit signatures
+For security & regulations compliance, commits must be cryptographically signed
+by [PGP](https://www.openpgp.org/)/[GPG](https://gnupg.org/), or SSH
+([since git v2.34](https://github.blog/2021-11-15-highlights-from-git-2-34/)).
+You can read more about this topic here:
+  - [Git's documentation](https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work)
+  - [Github's documentation](https://help.github.com/en/github/authenticating-to-github/signing-commits)
+  - [Gitlab's documentation](https://docs.gitlab.com/ee/user/project/repository/gpg_signed_commits/).
+  - [Signing Git commits with your SSH key](https://calebhearth.com/sign-git-with-ssh)
+
+### Commit messages
+
+Commit messages must be properly formatted (following the
+[Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) rules).
+The reasons behind this decision are many:
+  - The project's history has to be "easy" to read.
+  - It's easier to extract statistics from the commit logs.
+  - It's easier to generate useful changelogs.
+  - This practice enforces that committers think twice about the nature of their
+    contributions.
+  - It allows us to automate version numbering (following
+    [Semantic Versioning](https://semver.org/) rules)
+
+### Branch history
+
+The merge request's commits have to present a "clean" history, `git rebase` is
+your friend. This means:
+  - linear history
+  - commit messages matching what the commit does
+  - no "experimental" commits + their revert commits