Fix reference

Add reference to System.ComponentModel.DataAnnotation
Merge pull request #10 from odinn-b/patch-1

Fix reference
Update 7. Unit of Work.md
Update 8. Dependency Injection.md
Update 8. Dependency Injection.md
Update 8. Dependency Injection.md
Update 9. LINQ.md
Update 9. LINQ.md
Update 9. LINQ.md
Update and rename Unit testing.md to 11. Unit testing.md
Create 10. Mapping.md
Rename 11. Unit testing.md to 12. Unit testing.md
Create 11. Entity relationships.md
Update 10. Mapping.md
Create Validation.md
Create Security.md
Update and rename 11. Entity relationships.md to 10. Entity relationships.md
Update and rename 10. Mapping.md to 11. Mapping.md
Create 12. Fluent API.md
Update 12. Fluent API.md