[Docs] Add paragraph explaining where to find images when crafting render tests

Change-Id: I7b8c12ad22ed4d70218012448b8aa9f087a65238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835658
Commit-Queue: Ender <ender@google.com>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706201}

chrome/test/android/javatests/src/org/chromium/chrome/test/util/RENDER_TESTS.md

@@ -75,6 +75,29 @@ but if there appear to be goldens missing that should be there, try running
 `//chrome/test/data/android/manage_render_test_goldens.py download` to ensure
 that the downloaded goldens are current for the git revision.
 
+### Generating golden images locally
+
+New golden images may be downloaded from the trybots or retrieved locally. This
+section elaborates how to do the latter.
+
+You should always create your reference images on the same device type as the
+one running the tests. This is because each device/API version may produce a
+slightly different image, eg. due to different screen dimensions, DPI setting,
+or styling used across OS versions. This is also why each golden image name
+includes the device name and API version.
+
+When running a test with no goldens on the correct device, your tests should
+fail with an exception:
+
+```
+RenderTest Goldens missing for: <reference>. See RENDER_TESTS.md for how to fix this failure.
+```
+
+You will be able to find the images the device captured on the device's SD card.
+
+```
+adb -d shell ls /sdcard/chromium_tests_root/chrome/test/data/android/render_tests/failures
+```
 
 ## Implementation Details