File structure should include the 1x version too. (#581)

Its unclear in the docs that the 1x version should be included too. I believe that you do need it, correct me if I'm wrong?

docs/images.md

@@ -19,6 +19,7 @@ You can also use the `@2x` and `@3x` suffixes to provide images for different sc
 .
 ├── button.js
 └── img
+    ├── check.png
     ├── [email protected]
     └── [email protected]
 ```