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/docker/README.md

@@ -0,0 +1,10 @@
+# Docker
+
+This directory contains files for building Docker images for Danbooru.
+
+# External links
+
+* https://docs.docker.com/get-started/
+* https://docs.docker.com/engine/reference/commandline/cli/
+* https://docs.docker.com/engine/reference/builder/
+* https://hub.docker.com/r/evazion/danbooru