Add docs about context being needed

Author: TimDumol

README.md

@@ -6,14 +6,12 @@ Strawberry-sqlalchemy-mapper is the simplest way to implement autogenerated stra
 
 - Instead of manually listing every column and relationship in a SQLAlchemy model, strawberry-sqlalchemy-mapper
 lets you decorate a class declaration and it will automatically generate the necessary strawberry fields
-for all columns and relationships in the given model.
+for all columns and relationships (subject to the limitations below) in the given model.
 
 - Native support for most of SQLAlchemy's most common types.
-
 - Extensible to arbitrary custom SQLAlchemy types.
-
+- Automatic batching of queries, avoiding N+1 queries when getting relationships
 - Support for SQLAlchemy >=1.4.x
-
 - Lightweight and fast.
 
 ## Getting Started
@@ -45,6 +43,12 @@ strawberry_sqlalchemy_mapper = StrawberrySQLAlchemyMapper()
 class Employee:
     pass
 
+# context is expected to have an instance of StrawberrySQLAlchemyLoader
+class CustomGraphQLView(GraphQLView):
+    def get_context(self):
+        return {
+            "sqlalchemy_loader": StrawberrySQLAlchemyLoader(),
+        }
 
 # call finalize() before using the schema:
 strawberry_sqlalchemy_mapper.finalize()
@@ -62,7 +66,7 @@ Roots of polymorphic hierarchies are also expected to be registered via
 `strawberry_sqlalchemy_mapper.interface()`, and its concrete type and
 its descendants are expected to inherit from the interface:
 
-```
+```python
 class Book(Model):
     id = Column(UUID, primary_key=True)