Failed Jobs Plugin README.md

In this issue I have added a new `README.md` file to the Cockpit Failed Jobs plugin, to describe what it does and also how it has used the **[Camunda REST API](http://docs.camunda.org/latest/api-references/rest/)**.

I have tried to be more descriptive and provide more information about the related concepts such as **Jobs**, **The Job Executor**, **Job Definitions**, and **Incidents**.

How cockpit plugin engine allows us to create plugins with fewest lines of codes and how it registers the plugin into the view in **Angular MV model**, are other points that have been mentioned in the `README.md` file.