Update getting-started-with-entity-framework-core.md

add missing prerequisite

Author: bielu

16/umbraco-cms/tutorials/getting-started-with-entity-framework-core.md

@@ -28,6 +28,9 @@ Data stored in custom tables are not supported by default by add-ons such as Umb
 ## Prerequisite
 
 * An Umbraco project with content
+* If code created in separate project from Umbraco package, project has to reference:
+  * Microsoft.EntityFrameworkCore
+  * Microsoft.EntityFrameworkCore.Relational
 * EFCore CLI tool
   * Can be installed by running `dotnet tool install --global dotnet-ef` in the terminal