Update Readme with Builder (#8)

Author: douggynix

README.md

@@ -28,15 +28,20 @@ If we have to load a 'user' record from a database, we wouldn't want to send all
 This is where DTO Mapper Library comes handy.
 
 # Installation
+**_dto_mapper_** library depends on **_derive_builder_** to implement builder pattern for dto objects resulted from the dto mappers.
+By default, it generate builder for the dtos. 
 You can use this instruction to install the latest version of **dto_mapper** library to your project
 ```shell
+cargo add derive_builder
 cargo add dto_mapper
 ```
 And import it to the rust source file you need to use it:
 ```rust
 use dto_mapper::DtoMapper;
 ```
 
+More details on how to use derive_builder crate here: https://crates.io/crates/derive_builder
+
 # Example
 Let's say we want to create 3 special entities for our application
 - LoginDto that will contain only 2 fields from **User** such as _**username**_ and _**password**_. we would like to rename _**username**_ to _**login**_ in LoginDto
@@ -48,10 +53,16 @@ It takes only those lines below to get this work done. And the conversion are be
   ```rust
     use dto_mapper::DtoMapper;
 
+    /*** Use this declaration below in lib.rs if you're using a library crate , or in main.rs if you're using a binary crate.
+     if your crate has lib.rs and main.rs. Use it instead inside your lib.rs.
+    ***/
+    #[macro_use]
+    extern crate derive_builder;
+
     #[derive(DtoMapper,Default,Clone)]
     #[mapper( dto="LoginDto"  , map=[ ("username:login",true) , ("password",true)] , derive=(Debug, Clone, PartialEq) )]
     #[mapper( dto="ProfileDto" , ignore=["password"]  , derive=(Debug, Clone, PartialEq) )]
-    #[mapper( dto="PersonDto" , map=[ ("firstname",true), ("lastname",true), ("email",false) ]  )]
+    #[mapper( dto="PersonDto" , no_builder=true,  map=[ ("firstname",true), ("lastname",true), ("email",false) ]  )] //no_builder=true will not create default builder for that dto
     struct User{
         username: String,
         password: String,
@@ -97,6 +108,17 @@ Let's consider now we have a **PersonDto** and we'd like to convert it back part
         let user_from_person: User = person.into();
 ```
 
+Let's consider building LoginDto with the builder pattern object generated by **dto_mapper**:
+```rust
+        let mut login_dto_builder = LoginDtoBuilder::default();
+        let login_dto = login_dto_builder
+            .login("capois-lamort".into())
+            .password("hello123".into())
+            .build()
+            .expect("Failed to build login dto");
+        println!("LoginDto built with a builder: {:?}", login_dto);
+```
+
 Here is how **vscode** prints the code generated by DTO mapper for the **LoginDto** and **ProfileDto**
 ```Rust
 pub struct LoginDto {
@@ -138,6 +160,7 @@ struct SourceStruct{ }
        if `required_flag` is set to true, the destination dto field  will be exactly of the same type with the source one in the struct.
   - **Optional fields**
     - **ignore** : an array of fieldnames not to include in the destination dtos. `ignore=["field1", "field1"]`
-
       if **ignore** is present , then **map** field becomes optional. Except if needed rename destination fields for the dto
     - **derive** : a list of of macro to derive from. `derive=(Debug,Clone)`
+    - **no_builder**: a boolean flag to turn on or off builders for the dto. Default value is **_false_**. If the Dto name is "MyDto" , the builder will create a struct named "MyDtoBuilder" that can be used to build "MyDto" struct.
+