Added union docs. Fixed #493

docs/types/unions.rst

@@ -0,0 +1,63 @@
+Unions
+======
+
+Union types are very similar to interfaces, but they don't get
+to specify any common fields between the types.
+
+The basics:
+
+- Each Union is a Python class that inherits from ``graphene.Union``.
+- Unions don't have any fields on it, just links to the possible objecttypes.
+
+Quick example
+-------------
+
+This example model defines a ``Character`` interface with a name. ``Human``
+and ``Droid`` are two implementations of that interface.
+
+.. code:: python
+
+    import graphene
+
+    class Human(graphene.ObjectType):
+        name = graphene.String()
+        born_in = graphene.String()
+
+    class Droid(graphene.ObjectType):
+        name = graphene.String()
+        primary_function = graphene.String()
+
+    class Starship(graphene.ObjectType):
+        name = graphene.String()
+        length = graphene.Int()
+
+    class SearchResult(graphene.Union):
+        class Meta:
+            types = (Human, Droid, Starship)
+
+
+Wherever we return a SearchResult type in our schema, we might get a Human, a Droid, or a Starship.
+Note that members of a union type need to be concrete object types;
+you can't create a union type out of interfaces or other unions.
+
+The above types have the following representation in a schema:
+
+.. code::
+
+    type Droid {
+      name: String
+      primaryFunction: String
+    }
+
+    type Human {
+      name: String
+      bornIn: String
+    }
+
+    type Ship {
+      name: String
+      length: Int
+    }
+
+    union SearchResult = Human | Droid | Starship
+