Add help doc to fuzzer
Add help doc to fuzzer

BUG=skia:4438
TBR=jcgregorio

Review URL: https://codereview.chromium.org/1553173003

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot