Added to docs how to use library with custom headers (#34)

Author: gigitux

README.md

@@ -39,6 +39,25 @@ query = gql('''
 }
 ''')
 
+client.execute(query)
+```
+The example below shows how you can execute queries against a local schema with additional headers
+
+```python
+from gql import gql, Client
+from gql.transport.requests import RequestsHTTPTransport
+
+
+client = Client(transport = RequestsHTTPTransport(
+                                        url='/graphql',
+                                        headers={'Authorization': 'token'}), 
+                                        schema=schema)
+query = gql('''
+{
+  hello
+}
+''')
+
 client.execute(query)
 ```