Add README am: 3aeba4cab6 am: 591bf2432c am: f9cd093ca0

Original change: https://android-review.googlesource.com/c/platform/packages/apps/Messaging/+/2614297

Change-Id: I14048e3a7575f49da4cc945f32d2df839a4f0a00
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>
diff --git a/README b/README
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+This app is not actively supported and the source is only available as a reference. This project will be removed from the source manifest sometime in the future.
+