Use API Guardian annotations.

Change-Id: Icfd30e9b06e5c7ee9f51c6879bfe4978a6f44600
diff --git a/src/main/java/eu/mulk/jgvariant/core/Decoder.java b/src/main/java/eu/mulk/jgvariant/core/Decoder.java
index d0c0323..d30366e 100644
--- a/src/main/java/eu/mulk/jgvariant/core/Decoder.java
+++ b/src/main/java/eu/mulk/jgvariant/core/Decoder.java
@@ -12,6 +12,8 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Optional;
+import org.apiguardian.api.API;
+import org.apiguardian.api.API.Status;
 import org.jetbrains.annotations.Nullable;
 
 /**
@@ -42,6 +44,7 @@
  * @param <T> the type that the {@link Decoder} can decode.
  */
 @SuppressWarnings("java:S1610")
+@API(status = Status.EXPERIMENTAL)
 public abstract class Decoder<T> {
 
   private Decoder() {}