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This CL adds basic background, caveats, and basic instructions on adding
native resources to a DFM, and updates docs and examples re.
|load_native_on_get_impl|.

TBR=yfriedman@chromium.org

Bug: 1048740
Change-Id: I0c7adc21df568ce3ffc8895c81141bbe1db1850c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2069845
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744718}