✨(marketing) add documentation

Add documentation explainig how to use this module and how to add a new
backend.

documentation/how-to-use-marketing-backend.md

@@ -0,0 +1,49 @@
+# Using marketing backend
+
+If you need a marketing service in the application you are developping, this module will provide the implementation you need.
+
+The list of backends available and what they are able to do is subject to evolution.
+
+## Global idea
+
+A backend should inherit from `lasuite.marketing.backends.base.BaseBackend` class and implement the abstract methods declared in it.
+
+### Abstract methods
+
+- `def create_or_update_contact(self, contact_data: ContactData, timeout: int = None) -> dict:`: It is the method to call to create a new contact in your marketin system.
+    - `contact_data` is an instance of the `lasuite.marketing.ContactData` data class.
+    - `timeout` is an optional parameter used if you want to force a request timeout.
+
+### How to use a backend
+
+We provide a handler responsible to instantiate and configure a backend. You have to declare in your settings the backend to use with their needed parameters.
+
+The settings to use is `settings.LASUITE_MARKETING`. It is a `dict` containing two keys: `BACKEND` and `PARAMETERS`. The `BACKEND` is the full path to the backend class and the `PARAMETERS` is a dict containing all the parameters needed to instantiate the backend class.
+
+Example:
+
+```python
+settings.LASUITE_MARKETING = {
+    "BACKEND": "lasuite.marketing.backends.dummy.DummyBackend",
+    "PARAMETERS": {},
+}
+```
+
+Then to use the backend in your code, you have to import the `lasuite.marketing.marketing` and call the `create_or_update_contact` method.
+
+## Existing implementations
+
+### Dummy
+
+path: `lasuite.marketing.backend.dummy.DummyBackend`
+
+This implementation does nothing and accept no parameter.
+
+### Brevo
+
+path: ``lasuite.marketing.backend.brevo.BrevoBackend`
+parameters:
+    - `api_key`: The api_key used by the brevo client. Provided by brevo. Required
+    - `api_contact_list_ids`: The list of contact_list defined in brevo. At least one should be provided. Required
+    - `api_contact_attributes`: A dict of attributes to add to the contact. Optional
+