Add best practice for DTOs of modules.

docs/en/Best-Practices/Application-Services.md

@@ -26,6 +26,7 @@
 Example:
 
 ```c#
+[Serializable]
 public class IssueDto : FullAuditedEntityDto<Guid>
 {
     public string Title { get; set; }
@@ -34,6 +35,7 @@ public class IssueDto : FullAuditedEntityDto<Guid>
     public Collection<IssueLabelDto> Labels { get; set; }
 }
 
+[Serializable]
 public class IssueLabelDto
 {
     public Guid IssueId { get; set; }
@@ -54,6 +56,7 @@ public class IssueLabelDto
 Example:
 
 ````C#
+[Serializable]
 public class IssueWithDetailsDto : FullAuditedEntityDto<Guid>
 {
     public string Title { get; set; }
@@ -62,12 +65,14 @@ public class IssueWithDetailsDto : FullAuditedEntityDto<Guid>
     public Collection<LabelDto> Labels { get; set; }
 }
 
+[Serializable]
 public class MilestoneDto : EntityDto<Guid>
 {
     public string Name { get; set; }
     public bool IsClosed { get; set; }
 }
 
+[Serializable]
 public class LabelDto : EntityDto<Guid>
 {
     public string Name { get; set; }
@@ -129,6 +134,7 @@ Task<QuestionWithDetailsDto> CreateAsync(CreateQuestionDto questionDto);
 The related **DTO**:
 
 ````C#
+[Serializable]
 public class CreateQuestionDto
 {
     [Required]

docs/en/Best-Practices/Data-Transfer-Objects.md

@@ -5,3 +5,4 @@
 * **Do** define DTO members with **public getter and setter**.
 * **Do** use **data annotations** for **validation** on the properties of DTOs those are inputs of the service.
 * **Do** not add any **logic** into DTOs except implementing `IValidatableObject` when necessary.
+* **Do** mark all DTOs as **[Serializable]** since they are already serializable and developers may want to binary serialize them.