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/helpers/README.md

@@ -0,0 +1,25 @@
+# Helpers
+
+This directory contains helper functions used by views. Helpers are used for simple common functions, such as linking to
+users or formatting timestamps.
+
+Helper functions are globals. If you see an unnamespaced function in a view, and it's not a Rails function, then it's
+probably a helper defined here.
+
+All helper functions defined in this directory are globally available to all views. They're not limited to single views.
+For example, the functions in [posts_helper.rb](posts_helper.rb) are available to all views, not just to
+[app/views/posts](../views/posts).
+
+The use of helper functions should be minimized. Partials or components are preferred for more complex widgets, or for
+things used in only one or two places. Helper functions should be limited to very simple things used in nearly all
+views.
+
+# See also
+
+* [app/views](../views)
+* [app/components](../components)
+
+# External links
+
+* https://api.rubyonrails.org/classes/ActionController/Helpers.html
+* https://www.rubyguides.com/2020/01/rails-helpers/