Add Sync annotation.

Author: greenrobot-team

objectbox-java-api/src/main/java/io/objectbox/annotation/Sync.java

@@ -0,0 +1,17 @@
+package io.objectbox.annotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Enables sync for an {@link Entity} class.
+ * <p>
+ * Note that currently sync can not be enabled or disabled for existing entities.
+ * Also synced entities can not have relations to non-synced entities.
+ */
+@Retention(RetentionPolicy.CLASS)
+@Target(ElementType.TYPE)
+public @interface Sync {
+}