Add documentation for Jobber

jobber/README-short.txt

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+Jobber is an alternative to cron, with sophisticated status-reporting and error-handling.

jobber/content.md

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+# What is Jobber?
+
+Jobber is a utility for Unix-like systems that can run arbitrary commands, or "jobs", according to a schedule. It is meant to be a better alternative to the classic Unix utility cron.
+
+Along with the functionality of cron, Jobber also provides:
+
+-	**Job execution history:** you can see what jobs have recently run, and whether they succeeded or failed.
+-	**Sophisticated error handling:** you can control whether and when a job is run again after it fails. For example, after an initial failure of a job, Jobber can schedule future runs using an exponential backoff algorithm.
+-	**Sophisticated error reporting:** you can control whether Jobber notifies you about each failed run, or only about jobs that have been disabled due to repeated failures.
+
+# How to use this image
+
+This image contains Jobber running as an unprivileged user named "jobberuser". The jobs are defined in the file /home/jobberuser/.jobber. By default, the only job is one that prints "Jobber is running!" every second. You should replace it with your own jobs. Refer to [the documentation](https://dshearer.github.io/jobber/doc/v1.3/#jobfile) to learn how to do this.

jobber/github-repo

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+https://github.com/dshearer/jobber-docker
+

jobber/license.md

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+[Jobber's license](https://github.com/dshearer/jobber/blob/master/LICENSE)

jobber/maintainer.md

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+[Jobber](%%GITHUB-REPO%%)