update .gitignore

ref: https://github.com/gohugoio/hugo/blob/master/examples/blog/.gitignore

.gitignore

@@ -1 +1,12 @@
+# Hugo default output directory
+/public
+
+## OS Files
+# Windows
+Thumbs.db
+ehthumbs.db
+Desktop.ini
+$RECYCLE.BIN/
+
+# OSX
 .DS_Store