Reorganize gitignore file

So it's easier to follow.

.gitignore

@@ -4,32 +4,35 @@
 # or operating system, you probably want to add a global ignore instead:
 #   git config --global core.excludesfile '~/.gitignore_global'
 
-# Ignore bundler config.
-.bundle/
-
-# Ignore all logfiles and tempfiles.
-log/*
-!log/.keep
-tmp/
-
-/spec/examples.txt
+# Confidential information
 /config/database.yml
 /config/secrets.yml
 /config/deploy-secrets.yml
 /config/maintenance.yml
 
-/coverage/
+# Logfiles and tempfiles.
+log/*
+!log/.keep
+tmp/
 
-.byebug_history
-
-# Mac finder artifacts
-.DS_Store
-
-.ruby-gemset
-
-/public/sitemap.xml
-/public/assets/
-/public/machine_learning/data/
+# Files attached by users
 /public/system/
 /public/ckeditor_assets/
 /storage/
+
+# Files generated by scripts or compiled
+/public/sitemap.xml
+/public/assets/
+/public/machine_learning/data/
+
+# Bundler config, cache and gemsets
+.bundle/
+.ruby-gemset
+
+# Files used in local development
+.byebug_history
+.DS_Store
+
+# Test results
+/spec/examples.txt
+/coverage/