added the @NotNull annotation for parameters

src/main/java/amidst/documentation/NotNull.java

@@ -11,6 +11,6 @@ import java.lang.annotation.Target;
  */
 @Retention(RetentionPolicy.SOURCE)
 @Documented
-@Target(ElementType.METHOD)
+@Target({ ElementType.METHOD, ElementType.PARAMETER })
 public @interface NotNull {
 }

src/main/java/amidst/mojangapi/MojangApi.java

@@ -37,7 +37,7 @@ public class MojangApi {
 
 	public MojangApi(WorldBuilder worldBuilder,
 			DotMinecraftDirectory dotMinecraftDirectory,
-			VersionListJson versionList, File preferedJson) {
+			@NotNull VersionListJson versionList, File preferedJson) {
 		this.worldBuilder = worldBuilder;
 		this.dotMinecraftDirectory = dotMinecraftDirectory;
 		this.versionList = versionList;