Add video fetching instructions to the Vulkan README

Change-Id: I20fb4a5a4bbef023fba57db6dbf3f3689777a502

Author: lordalcol

external/vulkancts/README.md

@@ -73,6 +73,15 @@ With CMake out-of-source builds are always recommended. Create a build directory
 of your choosing, and in that directory generate Makefiles or IDE project
 using cmake.
 
+If you intend to run the Vulkan CTS video decode or encode tests, you must first
+download the required sample clips by running the two helper scripts in the
+`external/` directory:
+
+	python3 external/fetch_video_decode_samples.py
+	python3 external/fetch_video_encode_samples.py
+
+Each script will pull down the necessary video files into the CTS data tree.
+Both scripts support the `--help` flag to list all available options.
 
 ### Windows x86-32