docs: Add CONTRIBUTING.md guide

CONTRIBUTING.md

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+# Contributing to the Harmony project
+
+## Write small P-R
+
+Aim for the smallest piece of work that is mergeable.
+
+Mergeable means that :
+
+- it does not break the build
+- it moves the codebase one step forward
+
+P-Rs can be many things, they do not have to be complete features.
+
+### What a P-R **should** be
+
+- Introduce a new trait : This will be the place to discuss the new trait addition, its design and implementation
+- A new implementation of a trait : a new concrete implementation of the LoadBalancer trait
+- A new CI check : something that improves quality, robustness, ci performance
+- Documentation improvements
+- Refactoring
+- Bugfix
+
+### What a P-R **should not** be
+
+- Large. Anything over 200 lines (excluding generated lines) should have a very good reason to be this large.
+- A mix of refactoring, bug fixes and new features.
+- Introducing multiple new features or ideas at once.
+- Multiple new implementations of a trait/functionnality at once
+
+The general idea is to keep P-Rs small and single purpose.
+
+## Commit message formatting
+
+We follow conventional commits guidelines.
+
+https://www.conventionalcommits.org/en/v1.0.0/