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/webpack/base.js

@@ -13,6 +13,7 @@ module.exports = merge(webpackConfig, {
   },
   module: {
     rules: [{
+      // https://github.com/usabilityhub/rails-erb-loader
       test: /.erb$/,
       enforce: "pre",
       exclude: /node_modules/,