📝 Mention ATOM_HOME

docs/customizing-atom.md

@@ -100,6 +100,14 @@ namespaces: `core` and `editor`.
 
 You can open this file in an editor from the _Atom > Open Your Config_ menu.
 
+### Custom Configuration Location
+
+You can override the location that Atom stores configuration files and folders
+by setting the `ATOM_HOME` environment variable. The `ATOM_HOME` path will be
+used instead of `~/.atom` when it is set.
+
+This option can be useful when you want to make Atom portable across machines.
+
 ### Configuration Key Reference
 
 - `core`