Add docs to SKSubDesciptor

Summary: This tripped me up for a bit. Add some docs.

Reviewed By: Andrey-Mishanin

Differential Revision: D24160934

fbshipit-source-id: a36ac6b205a9edeeb0698f44476456583146907c

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutComponentKitSupport/SKSubDescriptor.h

@@ -13,5 +13,8 @@
  A SKSubDescriptor is a function which knows how to expose additional data
  to SKLayoutDescriptor. This class is for frameworks wanting to pass data
  along through the Layout Descriptor.
+
+ The infra expects that the string returned is a JSON string.
+ For example: @"{\'key\': 5}"
  */
 typedef NSString* (*SKSubDescriptor)(SKComponentLayoutWrapper* node);

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutComponentKitSupport/SKSubDescriptor.mm

@@ -1,6 +0,0 @@
-/*
- * Copyright (c) Facebook, Inc. and its affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- */