Add README for contrib/commands

contrib/commands/README.md

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+## Contributed Commands
+
+Although these commands 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 commands here, building a library of
+examples that might help others.
+
+I recommend *careful review* of any code from here before using it. No
+guarantees of code quality is assumed.