[fuzzer] Add docs about oss-fuzz dashboard and coverage report

Change-Id: I2ef9ee10a4e3b9c0c250b5ad2c6c99f9415b37db
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441098
Reviewed-by: John Stiles <johnstiles@google.com>
1 file changed