rls: add internal package description (#7009)

Author: creamsoup

rls/src/main/java/io/grpc/rls/internal/package-info.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2020 The gRPC Authors
+ *
+ * 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.
+ */
+
+/**
+ * Interfaces and implementations that are internal to gRPC.
+ *
+ * <p>All the content under this package and its subpackages are considered annotated with {@link
+ * io.grpc.Internal}.
+ */
+@io.grpc.Internal
+package io.grpc.internal;