Added missing package-info.java file (#3378)

server/src/main/java/org/opensearch/extensions/package-info.java

@@ -0,0 +1,15 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+/** 
+ * Main OpenSearch extensions package. OpenSearch extensions provide extensibility to OpenSearch.
+ * <p>
+ * The primary extension points are: 
+ *  <<enumerate the extension points and ideally give an example so users no how to extend OpenSearch>>>
+ */
+package org.opensearch.extensions;