Added Aluminum-specific directories to .gitignore

This commit adds the Aluminum preferences directory (/prefs/), resources directory (/resources/), and userscripts directory (/usr/) to .gitignore. This will enable users who install Aluminum via Git to adjust preferences and add their own files without these changes affecting the VCS.

.gitignore

@@ -103,5 +103,14 @@ dist
 # TernJS port file
 .tern-port
 
-# Sphinx documentation build folder (documentation is built automatically by Read the Docs---see https://readthedocs.org/ ---and is published automatically to https://aluminumjs.rtfd.io/)
+# Sphinx documentation build folder (documentation is automatically built and published by Read the Docs)
 docs/_build
+
+# Aluminum preferences folder
+prefs/
+
+# Aluminum resources folder
+resources/
+
+# Aluminum userscripts folder
+usr/