Add package-info file for docs (#3123)

Author: mikaelm12

clients/java/signalr/src/main/java/com/microsoft/signalr/package-info.java

@@ -0,0 +1,7 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+/**
+ * This package contains the classes for SignalR Java client.
+ */
+package com.microsoft.signalr;