Document how to make a new test data file public.
Document how to make a new test data file public.

If the "Public access" of a newly added test data file is "Not public",
"make testdata" will download a zero-length file and the SHA-1 checksum
check for the file will fail.

Change-Id: Ie399cfc758a65cffdb01a62eaa4a00b46d992b78

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom