Add left/right to editor & README.

src/localize/languages/en.json

@@ -80,6 +80,8 @@
           "modes": {
             "below": "Thumbnails below the media",
             "above": "Thumbnails above the media",
+            "left": "Thumbnails in a drawer left of the media",
+            "right": "Thumbnails in a drawer right of the media",
             "none": "No thumbnails"
           }
         },