Deprecate "crux" due to maintainer inactivity

crux/deprecated.md

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+This image is deprecated due to maintainer inactivity (last updated Aug 2015; [docker-library/official-images#1012](https://github.com/docker-library/official-images/pull/1012)).
+
+If a representative of the CRUX community would like to step up and continue maintenance, [the contribution guidelines](https://github.com/docker-library/official-images/blob/master/README.md#contributing-to-the-standard-library) are the best place to start.