Add extensions documentation

Author: leszekhanusz

docs/usage/extensions.rst

@@ -0,0 +1,36 @@
+.. _extensions:
+
+Extensions
+----------
+
+When you execute (or subscribe) GraphQL requests, the server will send
+responses which may have 3 fields:
+
+- data: the serialized response from the backend
+- errors: a list of potential errors
+- extensions: an optional field for additional data
+
+If there are errors in the response, then the
+:code:`execute` or :code:`subscribe` methods will
+raise a :code:`TransportQueryError`.
+
+If no errors are present, then only the data from the response is returned by default.
+
+.. code-block:: python
+
+    result = client.execute(query)
+    # result is here the content of the data field
+
+If you need to receive the extensions data too, then you can run the
+:code:`execute` or :code:`subscribe` methods with :code:`get_execution_result=True`.
+
+In that case, the full execution result is returned and you can have access
+to the extensions field
+
+.. code-block:: python
+
+    result = client.execute(query, get_execution_result=True)
+    # result is here an ExecutionResult instance
+
+    # result.data is the content of the data field
+    # result.extensions is the content of the extensions field

docs/usage/index.rst

@@ -11,3 +11,4 @@ Usage
    headers
    file_upload
    custom_scalars_and_enums
+   extensions