Add to README of common repo

Change-Id: Ibdac864ba04ec835ffc780fc2498ccfd7fa1d77b
Reviewed-on: https://skia-review.googlesource.com/c/common/+/413976
Reviewed-by: Eric Boren <borenet@google.com>

README

diff --git a/README b/README
index c6bc70c..9b85994 100644
--- a/README
+++ b/README
@@ -1,3 +1,9 @@
+NOTE: REPO IS UNUSED. IT IS ONLY KEPT AROUND BECAUSE OLDER VERSIONS OF SKIA
+STILL REFER TO IT IN THEIR DEPS.
+
+TODO(rmistry): Delete this repo after the branches that depend on it scroll out of
+the supported branches window.
+
 This repository contains code shared by the skia and skia-buildbot repositories.
 
 The file layout is done by language, such as the following directories