Create default.md (#370)

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* Update SUMMARY.md

* Update SUMMARY.md

* Update default.md

* Update default.md

* Update default.md

src/structs/field-shorthand.md

@@ -26,13 +26,44 @@ fn main() {
 
 *  The `new` function could be written using `Self` as a type, as it is interchangeable with the struct type name
 
-```rust,ignore
+     ```rust,editable
+     #[derive(Debug)]
+     struct Person {
+         name: String,
+         age: u8,
+     }
      impl Person {
          fn new(name: String, age: u8) -> Self {
              Self { name, age }
          }
      }
      ```    
+* Implement the `Default` trait for the struct. Define some fields and use the default values for the other fields.
+
+     ```rust,editable
+     #[derive(Debug)]
+     struct Person {
+         name: String,
+         age: u8,
+     }
+     impl Default for Person {
+         fn default() -> Person {
+             Person {
+                 name: "Bot".to_string(),
+                 age: 0,
+             }
+         }
+     }
+     fn create_default() {
+         let tmp = Person {
+             ..Default::default()
+         };
+         let tmp = Person {
+             name: "Sam".to_string(),
+             ..Default::default()
+         };
+     }
+     ```
 
 * Methods are defined in the `impl` block.
 * Use struct update syntax to define a new structure using `peter`. Note that the variable `peter` will no longer be accessible afterwards.