Use API Guardian annotations.

Change-Id: Icfd30e9b06e5c7ee9f51c6879bfe4978a6f44600
diff --git a/src/main/java/eu/mulk/jgvariant/core/Signature.java b/src/main/java/eu/mulk/jgvariant/core/Signature.java
index bb03b94..d9de5f1 100644
--- a/src/main/java/eu/mulk/jgvariant/core/Signature.java
+++ b/src/main/java/eu/mulk/jgvariant/core/Signature.java
@@ -6,6 +6,8 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
+import org.apiguardian.api.API;
+import org.apiguardian.api.API.Status;
 
 /**
  * A GVariant signature string.
@@ -24,6 +26,7 @@
  *       an array of 32-bit integers
  * </dl>
  */
+@API(status = Status.STABLE)
 public final class Signature {
 
   private final String signatureString;