Add archival message to tfjs-node. (#301)

Author: Nikhil Thorat

README.md

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+# This repository has been archived in favor of tensorflow/tfjs.
+
+This repo will remain around for some time to keep history but all future PRs should be sent to [tensorflow/tfjs](https://github.com/tensorflow/tfjs) inside the [tfjs-node](https://github.com/tensorflow/tfjs/tree/master/tfjs-node) folder.
+
+All history and contributions have been preserved in the monorepo.