Document enum mapping

I ran into the usage in an aspnet core article here https://docs.microsoft.com/en-us/aspnet/core/security/authorization/secure-data#modify-the-app-to-secure-user-data

There is no documentation in `/ef/core` regarding the usage of `enum`. A small article under `/ef/core/modeling/` would be great. Simply going over the fact that you can do this and how would suffice -- why you would do it (instead of using another DB table) might be out of scope, but would be nice to have.

Everything else is great, EF core is awesome!