Merge pull request #173 from petrus-v/patch-2

README example: add missing import

Author: Nabellaleen

README.md

@@ -38,6 +38,7 @@ class UserModel(Base):
 To create a GraphQL schema for it you simply have to write the following:
 
 ```python
+import graphene
 from graphene_sqlalchemy import SQLAlchemyObjectType
 
 class User(SQLAlchemyObjectType):