Merge "[sf] Add readme for surfaceflinger frontend" into udc-dev am: 0f0d7b03 am: d132d2cb

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/native/+/23272393



Change-Id: I8351837a6d74e6bd2e14d041b6fa52c91ad1e8c2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>