Add a marker artifact for the io.spring.javaformat gradle plugin

see https://docs.gradle.org/7.0/userguide/plugins.html#sec:plugin_markers
see spring-io#309

spring-javaformat-gradle/io.spring.javaformat.gradle.plugin/pom.xml

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+		 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<parent>
+		<groupId>io.spring.javaformat</groupId>
+		<artifactId>spring-javaformat-build</artifactId>
+		<version>0.0.32-SNAPSHOT</version>
+	</parent>
+	<groupId>io.spring.javaformat</groupId>
+	<artifactId>io.spring.javaformat.gradle.plugin</artifactId>
+	<packaging>pom</packaging>
+	<name>io.spring.javaformat Gradle Plugin Marker</name>
+	<dependencies>
+		<dependency>
+			<groupId>io.spring.javaformat</groupId>
+			<artifactId>spring-javaformat-gradle-plugin</artifactId>
+			<version>${project.version}</version>
+		</dependency>
+	</dependencies>
+</project>

spring-javaformat-gradle/pom.xml

@@ -32,5 +32,6 @@
 	</build>
 	<modules>
 		<module>spring-javaformat-gradle-plugin</module>
+		<module>io.spring.javaformat.gradle.plugin</module>
 	</modules>
 </project>