Add README.md

Change-Id: Icf78885f1e3ed647e1419d5b76ba41eb5b528a81
Reviewed-on: https://flutter-review.googlesource.com/c/third_party/libcxx/+/35004
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Nehal Patel <nehalvpatel@google.com>
Reviewed-by: Jesse Seales <jseales@google.com>

README.md

diff --git a/README.md b/README.md
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+# Mirrored from https://fuchsia.googlesource.com/third_party/libcxx
+
+This is a legacy mirror with the purpose of preserving the ability to build old versions of Flutter.
+
+Newer versions have already switched to https://llvm.googlesource.com/libcxx and the Fuchsia mirror
+is set to be archived and unavailable.