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/.

config/environments/README.md

@@ -0,0 +1,15 @@
+# Environments
+
+This directory contains environment-specific Rails configuration. This lets us have different
+configuration settings in development, test, and production modes.
+
+Universal Rails configuration goes in [config/application.rb](../application.rb) or in
+[config/initializers](../initializers).
+
+# See also
+
+* [config/application.rb](../application.rb)
+
+# External links
+
+* https://guides.rubyonrails.org/configuring.html#creating-rails-environments