add doc for extensions module

Author: sealkingking

README.md

@@ -59,6 +59,7 @@ Have a try with the settings dialog :stuck_out_tongue_winking_eye:
 * [Android Parcelable support](https://github.com/wuseal/JsonToKotlinClass/issues/44)
 * [Customize parent class declaration](https://github.com/wuseal/JsonToKotlinClass/issues/44)
 * Generating kotlin data class property order by alphabetical
+* Support customizing this plugin by `Extensions` module
 
 ### Generate Example 
 This is the example JSON from json.org

doc_for_extensions.md

@@ -0,0 +1,135 @@
+If you want to customize you own `JsonToKotlinClass` Plugin or want to contribute some new featrues for this project, This guide will help you.
+
+#### Customize Your Own Plugin
+
+Customize your own plugin by these steps, and you will have your own plugin, in these guide we will create a extension that will make all the property name to be all upper case
+
+1. Fork `JsonToKotlinClass` from Github
+
+2. Clone your forked project to local
+
+3. Open Project with Intellij Idea
+
+4. Create package under ..src/main/kotlin/extensions
+
+   ![image](https://user-images.githubusercontent.com/9211902/51250876-ccb8cb00-19d2-11e9-9620-9c45c8975bfa.png)
+
+   Package name could be your own domain name. in this case we will you `wu.seal`
+
+5. Create a Extension object class file under your created package
+
+   ![image](https://user-images.githubusercontent.com/9211902/51249232-a47a9d80-19cd-11e9-866f-b7e283da4853.png)
+
+6. Now the created file would like this:
+
+   ![image](https://user-images.githubusercontent.com/9211902/51249378-0fc46f80-19ce-11e9-8abd-0c926e8a726f.png)
+
+7. Add interfaces to be implemente
+
+   ![image](https://user-images.githubusercontent.com/9211902/51249511-75b0f700-19ce-11e9-9e74-ab014d3d77e3.png)
+
+8. Implement all the needed interface
+
+   ```kotlin
+   package extensions.wu.seal
+
+   import extensions.Extension
+   import wu.seal.jsontokotlin.classscodestruct.KotlinDataClass
+   import javax.swing.JPanel
+
+   object AllUpperCase :Extension(){
+
+       override fun createUI(): JPanel {
+
+       }
+
+       override fun intercept(kotlinDataClass: KotlinDataClass): KotlinDataClass {
+
+       }
+   }
+   ```
+
+9. Then we create a new UI for switching to enable all property name to be all upper case
+
+   ```kotlin
+    override fun createUI(): JPanel {
+
+           val configKey = "wu.seal.all_to_be_upper_case"
+
+           val checkBox = JCheckBox("Make all properties name to be all upper case").apply {
+               isSelected = getConfig(configKey).toBoolean()
+               addActionListener {
+                   setConfig(configKey, isSelected.toString())
+               }
+           }
+
+           return panel {
+               row {
+                   checkBox()
+               }
+           }
+
+       }
+   ```
+
+10. And make the properties names to be all upper case
+
+    ```kotlin
+     override fun intercept(kotlinDataClass: KotlinDataClass): KotlinDataClass {
+
+            //make all properties name to be all upper case
+            val newProperties = kotlinDataClass.properties.map { it.copy(name = it.name.toUpperCase()) }
+
+            return kotlinDataClass.copy(properties = newProperties)
+        }
+    ```
+
+    We use `map`method to create a new properties which all properties names are translate to be all upper case, And copy the incoming parameter `kotlinDataClass`  with replacing the new created properties, and return it to the system.
+
+11. Add the new object class into `ExtensionsCollector` object class which inside extensions package
+
+    ```kotlin
+    /**
+     * extension collect, all extensions will be hold by this class's extensions property
+     */
+    object ExtensionsCollector {
+        /**
+         * all extensions
+         */
+        val extensions = listOf(
+                PropertyPrefixSupport,
+                PropertySuffixSupport,
+                AllUpperCase
+        )
+    }
+    ```
+
+12. Run the plugin by clicking`runIde`task
+
+    ![image](https://user-images.githubusercontent.com/9211902/51250409-40f26f00-19d1-11e9-962c-7f44fb5549e4.png)
+
+    Then we will see the new IDE opened
+
+13. Create a new project in new opened IDE and create a new Kotlin File for inserting new Kotlin Data Class code, and open `JsonToKotlinClass` Plugin, Then we will see that new Extension was added in the extensions tab,Let's selected the new added checkbox and have a test if it works ok.
+
+    ![image](https://user-images.githubusercontent.com/9211902/51250670-15bc4f80-19d2-11e9-9cee-798489108adf.png)
+
+14. Then Let's see the tested result:
+
+    ![image](https://user-images.githubusercontent.com/9211902/51250749-5a47eb00-19d2-11e9-9824-c8f9ec0cc664.png)
+
+    ```kotlin
+    import com.google.gson.annotations.SerializedName
+    data class TestCode(
+        @SerializedName("hello")
+        val HELLO: String = ""
+    )
+    ```
+
+    Yeah! That what we want. Greate we have created the first our style plugin.
+
+    Any Question? Welcome to raise an issue [here](https://github.com/wuseal/JsonToKotlinClass/issues)
+
+    Demo could be found [here](https://github.com/wuseal/JsonToKotlinClass/tree/3.0-extension-demo)
+
+