add `--add` flag

pages/common/atom.md

@@ -11,6 +11,10 @@
 
 `atom -n {{path/to/file_or_folder}}`
 
+- Open a file or folder in an existing window:
+
+`atom -a {{path/to/file_or_folder}}`
+
 - Open atom in safe mode (does not load any additional packages):
 
 `atom --safe`