Document IEntity.GetKeys() method.

docs/Entities.md

@@ -45,11 +45,18 @@ public class UserRole : Entity
     {
             
     }
+    
+    public override object[] GetKeys()
+    {
+        return new object[] { UserId, RoleId };
+    }
 }
 ````
 
 For the example above, the composite key is composed of `UserId` and `RoleId`. For a relational database, it is the composite primary key of the related table.
 
+Entities with composite keys should implement the `GetKeys()` method as shown above.
+
 Notice that you also need to define keys of the entity in your **object-to-relational mapping** (ORM) configuration.
 
 > Composite primary keys has a restriction with repositories. Since it has not known Id property, you can not use `IRepository<TEntity, TKey>` for these entities. However, you can always use `IRepository<TEntity>`. See repository documentation (TODO: link) for more.