Add the missing SessionAttendee to Session 2 (#73)

Author: AndriySvyryd

docs/2. Build out BackEnd and Refactor.md

@@ -231,6 +231,27 @@ We're also going to take this opportunity to rename the `Models` directory in th
        }
    }
    ```
+ 1. Add an `SessionAttendee.cs` class with the following code:
+   ```csharp
+	using System;
+	using System.Collections.Generic;
+	using System.Linq;
+	using System.Threading.Tasks;
+
+	namespace BackEnd.Data
+	{
+		public class SessionAttendee
+		{
+			public int SessionID { get; set; }
+
+			public Session Session { get; set; }
+
+			public int AttendeeID { get; set; }
+
+			public Attendee Attendee { get; set; }
+		}
+	}
+   ```
 1. Add an `Attendee.cs` class with the following code:
    ```csharp
    using System;
@@ -242,7 +263,7 @@ We're also going to take this opportunity to rename the `Models` directory in th
        {
            public virtual ICollection<ConferenceAttendee> ConferenceAttendees { get; set; }
 
-           public virtual ICollection<Session> Sessions { get; set; }
+           public virtual ICollection<SessionAttendee> SessionsAttendees { get; set; }
        }
    }
    ```