Added a word about Service Factories in doc

docs/refcards/services.rst

@@ -43,6 +43,51 @@ Name            Type Description
 service.ranking int  The rank/priority of the service. The lower the rank, the more priority
 =============== ==== ===========================================================
 
+.. _service_factory:
+
+Service Factory
+---------------
+
+A service factory is a pseudo-service with a specific flag, which can create
+individual instances of service objects for different bundles.
+Sometimes a service needs to be differently configured depending on which
+bundle uses the service.
+For example, the log service needs to be able to print the logging bundle's id,
+otherwise the log would be hard to read.
+
+A service factory is registered in exactly the same way as a normal service,
+using :meth:`~pelix.framework.BundleContext.register_service`, with the
+``factory`` set to True``.
+The only difference is an indirection step before the actual service object is
+handed out.
+
+The client using the service need not, and should not, care if a service is
+generated by a factory or by a plain object.
+
+A simple service factory example
+
+.. code-block:: python
+
+    class LongFactory:
+        def get_service(self, bundle, registration):
+            """
+            Called each time a new bundle requires the service
+            """
+            return bundle.get_bundle_id()
+
+        def unget_service(self, bundle, registration):
+            """
+            Called when a bundle has released all its references
+            to the service
+            """
+            # Release connections, ...
+            pass
+
+    bundle_context.register_service("long", LongFactory(), {}, factory=True)
+
+.. note:: The framework will cache generated service objects.
+   Thus, at most one service can be generated per client bundle.
+
 API
 ---