Add contributing guidelines

CONTRIBUTING.md

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+# Contributing Guidelines
+
+First off, thanks for considering to contribute to this project!
+
+These are mostly guidelines, not rules. Use your best judgment, and feel free to propose changes to this document in a pull request.
+
+## Git flow
+
+### Creating branches
+
+A new branch should be always created from the main branch `master`, except in certain cases which require to be justified.
+
+#### Naming pattern
+
+The pattern is: `{category}/{slugified-description}`. Where:
+
+- `category` is the type of work. Can be: `feature`, `bug`, `tooling`, `refactor`, `test`, `chore`, `release`, `hotfix`, `docs`, `ci`, `deploy` or `release-candidate`.
+- `slugified-description` is the description of the work, slugified.
+
+Example: `feature/improve-encoding`
+
+### Merge Requests workflow
+
+#### Rules
+
+- the code coverage must be increased or equal, never decreased. If you write some new code, write new tests.
+- the code must run without any error on the CI.
+
+#### Using the draft status
+
+A draft Merge Request is a merge request that is not ready to be merged but the code is published to allow other team mates follow the development.
+
+Comments are welcome but they must be global, about the conception, not the details (wait for the WIP status removal).
+
+---
+
+## Code Style
+
+Make sure your code _roughly_ follows [PEP-8](https://www.python.org/dev/peps/pep-0008/) and keeps things consistent with the rest of the code. Related tools:
+
+- docstrings: [sphinx-style](https://sphinx-rtd-tutorial.readthedocs.io/en/latest/docstrings.html#the-sphinx-docstring-format) is used to write technical documentation.
+- formatting: [black](https://black.readthedocs.io/) is used to automatically format the code without debate.
+- sorted imports: [isort](https://pycqa.github.io/isort/) is used to sort imports
+- static analisis (linter): [flake8](https://flake8.pycqa.org/) is used to catch some dizziness and keep the source code healthy.
+
+---
+
+## Git hooks
+
+We use git hooks through [pre-commit](https://pre-commit.com/) to enforce and automatically check some "rules". Please install it before to push any commit: `pre-commit install`.
+
+See the relevant configuration file: `.pre-commit-config.yaml`.