Add README.md to NNAPI fuzzing directory

Bug: N/A
Test: N/A
Change-Id: I63a56d9b336513bc3c21016f8ae3ca4314cf1659
Merged-In: I63a56d9b336513bc3c21016f8ae3ca4314cf1659
(cherry picked from commit aeaa088279d299e0f8ae7895d67b25234f05a7bd)
1 file changed