Add the "Multi-Release: true" attribute to built jar

Reminder that the multiReleaseJar is a separate task from the jar task.
This plugin does not mutate the jar task in any capacity.

Author: Jonathing

build.gradle

@@ -62,7 +62,7 @@ tasks.named('shadowJar', ShadowJar) {
 }
 
 changelog {
-    fromBase()
+    from '0.1'
 }
 
 gradlePlugin {

src/main/java/net/minecraftforge/multi_release/gradle/internal/MultiReleaseContainerImpl.java

@@ -18,6 +18,7 @@
 import org.gradle.api.component.SoftwareComponentFactory;
 import org.gradle.api.file.ArchiveOperations;
 import org.gradle.api.file.DuplicatesStrategy;
+import org.gradle.api.java.archives.Manifest;
 import org.gradle.api.model.ObjectFactory;
 import org.gradle.api.provider.Provider;
 import org.gradle.api.provider.ProviderFactory;
@@ -74,6 +75,8 @@ private TaskProvider<Jar> createJar() {
 
             task.with(jar.get());
             task.setManifest(jar.get().getManifest());
+
+            task.manifest(Closures.<Manifest>consumer(manifest -> manifest.getAttributes().put("Multi-Release", "true")));
         });
     }