Show alternative method to define database URI

Show an alternative method for defining database URI on the README file, 
since probably many users do not want to set the URI through the command line.

Author: igortg

README.md

@@ -37,3 +37,10 @@ def test_connection(connection):
 You need to provide the connection URL for the engine when invoking the pytest command::
 
     pytest --sqlalchemy-connect-url="postgresql://scott:tiger@localhost:5432/mydatabase"
+    
+Or override the `sqlalchemy_connect_url` fixture on your conftest file:
+
+    @pytest.fixture()
+    def sqlalchemy_connect_url():
+        return 'postgresql://scott:tiger@localhost:5432/mydatabase'
+