better backend api documentation

docs/backend_api.rst

@@ -1,8 +1,54 @@
-Package ldapcherry.backend
-==========================
+Implementing your own backend
+=============================
+
+API
+~~~
+
+To create your own backend, you must implement the following API:
 
 .. automodule:: ldapcherry.backend
     :members:
     :undoc-members:
     :show-inheritance:
 
+Configuration
+~~~~~~~~~~~~~
+
+Configuration for your backend is declared in the main ini file, inside [backends] section:
+
+For example with the configuration:
+
+.. sourcecode:: ini
+
+    [backends]
+
+    # class path to module
+    b_id.module = "my.backend.module"
+
+    b_id.param1 = "my value 1"
+    b_id.param2 = "my value 2"
+
+.. note::
+
+    One module can be instanciated several times, the prefix b_id permits
+    to differenciate instances and their specific configuration.
+
+The following hash will be passed as configuration to the module constructor as parameter config:
+
+.. sourcecode:: python
+
+    {
+        'param1': "my value 1",
+        'param2': "my value 2",
+    }
+
+Exceptions
+~~~~~~~~~~
+The following exception can be used in your module
+
+*
+*
+*
+*
+
+These exceptions permit a nicer error handling and avoid a generic message to be thrown at the user.