Mention the serialization plugin in the GRADLE_PLUGIN.md (#2217)

GRADLE_PLUGIN.md

@@ -335,6 +335,25 @@ components.main {
 
 </div>
 
+### Serialization plugin
+
+The plugin is shipped with a customized version of the `kotlinx.serialization` plugin. To use it you don't have to 
+add new buildscript dependencies, just apply the plugins and add a dependency on the serialization library:
+
+<div class="sample" markdown="1" theme="idea" mode="groovy">
+
+```groovy
+apply plugin: 'org.jetbrains.kotlin.platform.native'
+apply plugin: 'kotlinx-serialization-native'
+
+dependencies {
+    implementation 'org.jetbrains.kotlinx:kotlinx-serialization-runtime-native'
+}
+```
+
+</div>
+
+The the [example project](https://github.com/ilmat192/kotlin-native-serialization-sample) for details.
 
 ### DSL example