Adds a README file for the scripts used to create gold instances as suggested here:
Adds a README file for the scripts used to create gold instances as suggested here:

https://codereview.chromium.org/1251383002/

BUG=skia:

Review URL: https://codereview.chromium.org/1266013008

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot