docs: document config/ and some directories in app/.

* Add README files to several directories in app/ giving a brief
  overview of some parts of Danbooru's architecture.
* Add documentation for files in config/.

app/jobs/README.md

@@ -0,0 +1,58 @@
+# Jobs
+
+This directory contains background jobs used by Danbooru. Jobs are used to
+handle slow-running tasks that need to run in the background, such as processing
+uploads or bulk update requests. They're also used for asynchronous tasks, such
+as sending emails, that may temporarily fail but can be automatically retried
+later.
+
+Jobs use the Rails Active Job framework. Active Job is a common framework that
+allows jobs to be run on different job runner backends.
+
+In the production environment, jobs are run using the Delayed Job backend.  Jobs
+are stored in the database in the `delayed_job` table. Worker processes spawned
+by `bin/delayed_job` poll the table for new jobs to work.
+
+In the development environment, jobs are run with an in-process thread pool.
+This will run jobs in the background, but will drop jobs when the server is
+restarted.
+
+There are two job queues, the `default` queue and the `bulk_update`. The
+`bulk_update` queue handles bulk update requests. It has only one worker so that
+bulk update requests are effectively processed sequentially. The `default` queue
+handles everything else.
+
+There is a very minimal admin dashboard for jobs at https://danbooru.donmai.us/delayed_jobs.
+
+Danbooru also has periodic maintenance tasks that run in the background as cron
+jobs. These are different from the jobs in this directory. See [app/logical/danbooru_maintenance.rb](../logical/danbooru_maintenance.rb).
+
+# Usage
+
+Start a pool of job workers:
+
+```
+RAILS_ENV=production bin/delayed_job --pool=default:8 --pool=bulk_update start
+```
+
+# Examples
+
+Spawn a job to be worked in the background. It will be worked as soon as a
+worker is available:
+
+```ruby
+DeleteFavoritesJob.perform_later(user)
+```
+
+# See also
+
+* [app/logical/danbooru_maintenance.rb](../logical/danbooru_maintenance.rb)
+* [app/controllers/delayed_jobs_controller.rb](../controllers/delayed_jobs_controller.rb)
+* [config/initializers/delayed_jobs.rb](../../config/initializers/delayed_jobs.rb)
+* [test/jobs](../../test/jobs)
+
+# External links
+
+* https://guides.rubyonrails.org/active_job_basics.html
+* https://github.com/collectiveidea/delayed_job
+* https://danbooru.donmai.us/delayed_jobs