Add initial background jobs doc.

docs/Background-Jobs.md

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+## Background Jobs
+
+### Introduction
+
+Background jobs are used to queue some tasks to be executed in the background. You may need background jobs for several reasons. Here are some examples:
+
+- To perform **long-running tasks** without having the users wait. For example, a user presses a 'report' button to start a long-running reporting job. You add this job to the **queue** and send the report's result to your user via email when it's completed.
+- To create **re-trying** and **persistent tasks** to **guarantee** that a code will be **successfully executed**. For example, you can send emails in a background job to overcome **temporary failures** and **guarantee** that it eventually will be sent. That way users do not wait while sending emails.
+
+Background jobs are **persistent** that means they will be **re-tried** and **executed** later even if your application crashes.
+
+ABP provides an **abstraction** module and **several implementations** for background jobs. It has a built-in/default implementation as well as Hangfire and RabbitMQ integrations.
+
+### Abstraction Module
+
+TODO
+
+### Creating a Background Job
+
+A background job is a class that implements the `IBackgroundJob<TArgs>` interface or derives from the `BackgroundJob<TArgs>` class. `TArgs` is a simple plain C# class to store the job data.
+
+An example background job to send emails in background:
+