README: document archiving and direct issues to gonum/gonum

README.md

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 # This repository is no longer maintained. Development has moved to https://github.com/gonum/gonum.
 
 mathext implements basic elementary functions not included in the Go standard library
+
+## Issues
+
+If you find any bugs, feel free to file an issue on the github [issue tracker for gonum/gonum](https://github.com/gonum/gonum/issues) if the bug exists in that reposity; no code changes will be made to this repository. Other dicussions should be taken to the gonum-dev Google Group.
+
+https://groups.google.com/forum/#!forum/gonum-dev