Placeholder for sky/engine/README.md.

TBR=eseidel

Review URL: https://codereview.chromium.org/1198323003.

engine/README.md

+Sky Engine
+==========
+
+This directory contains the bulk of the C++ code backing the
+``dart:sky`` module.
+
+It is based on Blink, Chromium's rendering engine, which itself is
+descended from WebKit, which is descended from KHTML.