Migrate the Security Sheriff documentation to Markdown.

Bug: None
Change-Id: I07396ea6215eb72fb4f398877c901b2e4deeab5f
Reviewed-on: https://chromium-review.googlesource.com/742742
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Martin Barbella <mbarbella@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512715}
4 files changed