Added registry docs

Author: syrusakbary

docs/registry.rst

@@ -0,0 +1,53 @@
+Graphene-Django Registry
+========================
+
+Graphene-Django uses a Registry to keep track of all the Django Models
+and the ``DjangoObjectTypes`` associated to them.
+
+This way, we make the library smart enough to convert automatically the 
+relations between models to Graphene fields automatically (when possible).
+
+
+get_global_registry
+-------------------
+
+By default, all model/objecttype relations will live in the global registry.
+
+.. code:: python
+
+    from graphene_django.registry get_global_registry
+
+    class Reporter(DjangoObjectType):
+        '''Reporter description'''
+        class Meta:
+            model = ReporterModel
+
+    global_registry = get_global_registry
+    global_registry.get_type_for_model(ReporterModel) # == Reporter
+
+
+One Model, multiple types
+-------------------------
+
+There will be some cases where we need one Django Model to
+have multiple graphene ``ObjectType``s associated to it.
+
+.. code:: python
+
+    from graphene_django.registry import Registry
+
+    class Reporter(DjangoObjectType):
+        '''Reporter description'''
+        class Meta:
+            model = ReporterModel
+
+    class Reporter2(DjangoObjectType):
+        '''Reporter2 description'''
+        class Meta:
+            model = ReporterModel
+            skip_global_registry = True
+            # We can also specify a custom registry with
+            # registry = Registry()
+
+This way, the ``ReporterModel`` could have two different types living in the same
+schema.