Add marker artifact to gradle plugin (#59)

* Setup gradle plugin via java-gradle-plugin

This adds a marker artifact to the publication

* Update readme with plugins block example

README.md

@@ -22,18 +22,13 @@ We recommend you look at some existing projects for examples:
 - [gdx-video-desktop](https://github.com/libgdx/gdx-video/blob/master/gdx-video-desktop/build.gradle)
 - [Jamepad](https://github.com/libgdx/Jamepad/blob/master/build.gradle)
 
-```
-// Add buildscript dependency
-buildscript {
-    dependencies {
-        classpath "com.badlogicgames.gdx:gdx-jnigen-gradle:2.X.X"
-    }
-}
-
+```gradle
 // Apply jnigen plugin
-apply plugin: "com.badlogicgames.gdx.gdx-jnigen"
+plugins {
+    id "com.badlogicgames.gdx.gdx-jnigen"
+}
 
-...
+// ...
 
 // Define jnigen extension
 jnigen {

gdx-jnigen-gradle/build.gradle

@@ -1,4 +1,17 @@
+plugins {
+    id 'java-gradle-plugin'
+}
+
 dependencies {
     implementation project(":gdx-jnigen")
     implementation gradleApi()
-}
+}
+
+gradlePlugin {
+    plugins {
+        register('gdxJniGen') {
+            id = 'com.badlogicgames.gdx.gdx-jnigen'
+            implementationClass = 'com.badlogic.gdx.jnigen.gradle.JnigenPlugin'
+        }
+    }
+}