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/mailers/README.md

@@ -0,0 +1,31 @@
+# Mailers
+
+This directory contains mailers for sending emails. Mailers are kind of like controllers, except for generating emails
+instead of generating HTML.
+
+The actual email templates live in [app/views/user_mailer](../views/user_mailer).
+
+Emails are sent asynchronously using a background job. If sending the email fails, it will be retried later.
+
+Sending emails requires a SMTP server to be configured in
+[config/danbooru_local_config.rb](../../config/danbooru_local_config.rb). In production,
+[Amazon SES](https://aws.amazon.com/ses/) is used to send emails.
+
+Email templates can be previewed at http://localhost:3000/rails/mailers (assuming you're running `bin/rails server` on
+port 3000, the default).
+
+# Example
+
+```ruby
+UserMailer.welcome_user(@user).deliver_later
+```
+
+# See also
+
+* [app/views/user_mailer](../views/user_mailer)
+* [test/mailers/previews/user_mailer_preview.rb](../../test/mailers/previews/user_mailer_preview.rb)
+
+# External links
+
+* https://guides.rubyonrails.org/action_mailer_basics.html
+* https://guides.rubyonrails.org/testing.html#testing-your-mailers