Update DESIGN.md with info on user logins.
Update DESIGN.md with info on user logins.

BUG=skia:
R=stephana@google.com, rmistry@google.com

Author: jcgregorio@google.com

Review URL: https://codereview.chromium.org/594253002

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