Update CONTRIBUTING.md

Author: dmiller

CONTRIBUTING.md

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+This is a [Clojure contrib] project.
+
+Under the Clojure contrib [guidelines], this project cannot accept
+pull requests. All patches must be submitted via [JIRA].
+
+See [Contributing] on the Clojure website for
+more information on how to contribute.
+
+[Clojure contrib]: https://clojure.org/community/contrib_libs
+[Contributing]: https://clojure.org/community/contributing
+[JIRA]: https://clojure.atlassian.net/browse/TNS
+[guidelines]: https://clojure.org/community/contrib_howto