add app context error message to docs

add link to docs to error message
fixes pallets-eco#553

Author: davidism

docs/contexts.rst

@@ -33,11 +33,17 @@ Because there might be more than one application created.
 
 So how does :class:`SQLAlchemy` come to know about your application?
 You will have to setup an application context.  If you are working inside
-a Flask view function, that automatically happens.  However if you are
-working inside the interactive shell, you will have to do that yourself
-(see `Creating an Application Context
+a Flask view function or a CLI command, that automatically happens. However,
+if you are working inside the interactive shell, you will have to do that
+yourself (see `Creating an Application Context
 <http://flask.pocoo.org/docs/appcontext/#creating-an-application-context>`_).
 
+If you try to perform database operations outside an application context, you
+will see the following error:
+
+    No application found. Either work inside a view function or push an
+    application context.
+
 In a nutshell, do something like this:
 
 >>> from yourapp import create_app

flask_sqlalchemy/__init__.py

@@ -897,7 +897,8 @@ def get_app(self, reference_app=None):
 
         raise RuntimeError(
             'No application found. Either work inside a view function or push'
-            ' an application context.'
+            ' an application context. See'
+            ' http://flask-sqlalchemy.pocoo.org/contexts/.'
         )
 
     def get_tables_for_bind(self, bind=None):