Clarify property/field mapping for read-only properties/fields

The document explains how to use the NotMapped attribute or the .Ignore fluent method to exclude public properties. How do we include (map) private properites?

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#### Document Details

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* ID: 76c24b57-557f-a6d6-16f1-1da50e4744cd
* Version Independent ID: 8bee29c6-58e4-1d07-025e-0bdcead9b5a1
* Content: [Including & Excluding Properties - EF Core](https://docs.microsoft.com/en-us/ef/core/modeling/included-properties)
* Content Source: [entity-framework/core/modeling/included-properties.md](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/modeling/included-properties.md)
* Product: **entity-framework**
* GitHub Login: @rowanmiller
* Microsoft Alias: **divega**