Add README am: 3aeba4ca

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



Change-Id: Ic8def036efb311ef3cc7f1e25a68c4b3ce211efb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

README

+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.
+