[fuzzer] Add docs about testing oss-fuzz locally.
[fuzzer] Add docs about testing oss-fuzz locally.

Change-Id: Ib1e94b3924ece8b249a42a5c25daf771132e534a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441058
Reviewed-by: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia