Add database context section to SQL tutorial

Added section on working with the database context.

Author: wadepickett

aspnetcore/tutorials/first-mvc-app/working-with-sql/includes/working-with-sql7.md

@@ -17,6 +17,8 @@ By the end of this guide, you’ll understand the basics of working with databas
 
 This tutorial uses a database you set up in the previous step: <xref:tutorials/first-mvc-app/adding-model>.
 
+## Working with the database context
+
 The `MvcMovieContext` object handles the task of connecting to the database and mapping `Movie` objects to database records. The database context is registered with the [Dependency Injection](xref:fundamentals/dependency-injection) container in the `Program.cs` file:
 
 # [Visual Studio](#tab/visual-studio)