Documentation for Include rules

Author: JeremySkinner

docs/_doc/start.md

@@ -14,5 +14,6 @@ sections:
   - /start/collections
   - /start/complex-properties
   - /start/rulesets
+  - /start/including-rules
   - /start/configuring
 ---

docs/_doc/start/including-rules.md

@@ -0,0 +1,36 @@
+---
+title: Including Rules
+---
+You can include rules from other validators provided they validate the same type. This allows you to split rules across multiple classes and compose them together (in a similar way to how other languages support traits). For example, imagine you have 2 validators that validate different aspects of a `Person`:
+
+```csharp
+public class PersonAgeValidator : AbstractValidator<Person>  {
+  public PersonAgeValidator() {
+    RuleFor(x => x.DateOfBirth).Must(BeOver18);
+  }
+
+  protected bool BeOver18(DateTime date) {
+    //...
+  }
+}
+
+public class PersonNameValidator : AbstractValidator<Person> {
+  public PersonNameValidator() {
+    RuleFor(x => x.Surname).NotNull().Length(0, 255);
+    RuleFor(x => x.Forename).NotNull().Length(0, 255);
+  }
+}
+```
+
+Because both of these validators are targetting the same model type (`Person`), you can combine them using `Include`:
+
+```csharp
+public class PersonValidator : AbstractValidator<Person> {
+  public PersonValidator() {
+    Include(new PersonAgeValidator());
+    Include(new PersonNameValidator());
+  }
+}
+```
+
+*Note*: You can only include validators that target the same type as the root validator.