vulkan: Document codegen instructions and code style

Change-Id: I7565506a866b72f4ce31ac6bdd3d97e357497ab0
(cherry picked from commit e0d27bb1f3671401c2f6d84a60111f5c695c97d3)
diff --git a/vulkan/libvulkan/dispatch_gen.h b/vulkan/libvulkan/dispatch_gen.h
index 14c5da8..14a4b95 100644
--- a/vulkan/libvulkan/dispatch_gen.h
+++ b/vulkan/libvulkan/dispatch_gen.h
@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+// WARNING: This file is generated. See ../README.md for instructions.
+
 #define VK_USE_PLATFORM_ANDROID_KHR
 #include <vulkan/vk_android_native_buffer.h>
 #include <vulkan/vk_ext_debug_report.h>