Add contrib/hooks

contrib/hooks/README.md

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+## Contributed Hooks
+
+Although these [hooks][hooks-help] are available as part of the official
+**yadm** source tree, they have a somewhat different status. The intention is to
+keep interesting and potentially useful hooks here, building a library of
+examples that might help others.
+
+In some cases, an experimental new feature can be build entirely with hooks, and
+this is a place to share it.
+
+I recommend *careful review* of any code from here before using it. No
+guarantees of code quality is assumed.
+
+[hooks-help]: https://github.com/TheLocehiliosan/yadm/blob/master/yadm.md#hooks