Add Experimental annotation.

onebusaway-gtfs/src/main/java/org/onebusaway/gtfs/annotations/Experimental.java

@@ -0,0 +1,36 @@
+/**
+ * Copyright (C) 2024 OneBusAway contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onebusaway.gtfs.annotations;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+import java.lang.annotation.Documented;
+
+/**
+ * This annotation indicates that a field is experimental.
+ */
+@Retention(value = RetentionPolicy.SOURCE)
+@Target(value = ElementType.FIELD)
+@Documented
+public @interface Experimental {
+  /**
+   * This indicates what issue this field was proposed in.
+   * Ideally this should be a URL.
+   */
+  String proposedBy();
+}