add CONTRIBUTING.md

CONTRIBUTING.md

@@ -1,9 +1,14 @@
 # Contributing
 
-Commit messages should be written in English.
+When contributing to this repository, please first discuss the change you wish to make via issue. We will than discuss the best way to implement it.
 
-We only accept contributions if they are *signed off* with the contributor's real name.
+## Code rules
 
-By signing off their contributions, contributors accept the [developer certificate of origin](https://developercertificate.org).
+### Backend (API)
 
-If you wish, you can also send your suggestions directly to `contact@code.gouv.fr`.
+The buisness logic is in `api/src/core`. Which is split in different parts:
+
+- `api/src/core/usecases`: contains the usecases that are called by the router. This orchestrates the flow of business logic. It should not be specific to the sources, and it should always use a port when accessing a source.
+- `api/src/core/helpers`: contains the helpers that are used by the usecases. This can contain all the reusable business logic.
+- `api/src/core/ports`: the contracts to discuss with the outside world and infrastructure (database, external APIs, etc.)
+- `api/src/core/adapters`: the implementations of the ports. This is the only place where we should have code specific to the sources for example.