Added a message to a initial migrate.

docs/index.rst

@@ -47,8 +47,8 @@ This will add a `migrations` folder to your application. The contents of this fo
 
 You can then generate an initial migration::
 
-    $ flask db migrate
-    
+    $ flask db migrate -m "Initial migration."
+
 The migration script needs to be reviewed and edited, as Alembic currently does not detect every change you make to your models. In particular, Alembic is currently unable to detect table name changes, column name changes, or anonymously named constraints. A detailed summary of limitations can be found in the `Alembic autogenerate documentation <http://alembic.zzzcomputing.com/en/latest/autogenerate.html#what-does-autogenerate-detect-and-what-does-it-not-detect>`_. Once finalized, the migration script also needs to be added to version control.
 
 Then you can apply the migration to the database::