Add documentation for relay mutations.

docs/pages/docs/relay.md

@@ -54,6 +54,28 @@ class Query(graphene.ObjectType):
     node = relay.NodeField()
 ```
 
+## Mutations
+
+Most APIs don't just allow you to read data, they also allow you to write. In GraphQL, this is done using mutations. Just like queries, Relay puts some additional requirements on mutations, but Graphene nicely manages that for you. All you need to do is make your mutation a subclass of `relay.ClientIDMutation`.
+
+```python
+class IntroduceShip(relay.ClientIDMutation):
+
+    class Input:
+        ship_name = graphene.String(required=True)
+        faction_id = graphene.String(required=True)
+
+    ship = graphene.Field(Ship)
+    faction = graphene.Field(Faction)
+
+    @classmethod
+    def mutate_and_get_payload(cls, input, info):
+        ship_name = input.get('ship_name')
+        faction_id = input.get('faction_id')
+        ship = create_ship(ship_name, faction_id)
+        faction = get_faction(faction_id)
+        return IntroduceShip(ship=ship, faction=faction)
+```
 
 ## Useful links