Added docs for List and NonNull types. Fixed #326

Author: syrusakbary

docs/types/index.rst

@@ -7,6 +7,7 @@ Types Reference
 
    enums
    scalars
+   list-and-nonnull
    interfaces
    abstracttypes
    objecttypes

docs/types/list-and-nonnull.rst

@@ -0,0 +1,50 @@
+Lists and Non-Null
+==================
+
+Object types, scalars, and enums are the only kinds of types you can
+define in Graphene. But when you use the types in other parts of the
+schema, or in your query variable declarations, you can apply additional
+type modifiers that affect validation of those values.
+
+NonNull
+-------
+
+.. code:: python
+
+    import graphene
+
+    class Character(graphene.ObjectType):
+        name = graphene.NonNull(graphene.String)
+
+
+Here, we're using a ``String`` type and marking it as Non-Null by wrapping
+it using the ``NonNull`` class. This means that our server always expects
+to return a non-null value for this field, and if it ends up getting a
+null value that will actually trigger a GraphQL execution error,
+letting the client know that something has gone wrong.
+
+
+The previous ``NonNull`` code snippet is also equivalent to:
+
+.. code:: python
+
+    import graphene
+
+    class Character(graphene.ObjectType):
+        name = graphene.String(required=True)
+
+
+List
+----
+
+.. code:: python
+
+    import graphene
+
+    class Character(graphene.ObjectType):
+        appears_in = graphene.List(graphene.String)
+
+Lists work in a similar way: We can use a type modifier to mark a type as a
+``List``, which indicates that this field will return a list of that type.
+It works the same for arguments, where the validation step will expect a list
+for that value.