Add a disclaimer

Author: gaearon

README.md

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+# Discontinued
+
+Starting with React 16.1.0, **we will no longer be publishing new releases on Bower.**
+
+You can continue using Bower for old releases, or point your Bower configs to the [React UMD builds hosted on unpkg](https://reactjs.org/docs/installation.html#using-a-cdn).
+
+Those builds mirror npm releases and will continue to be updated.