Add image cropping parameter to wiki

Volume-Server-API.md

@@ -10,6 +10,10 @@ GET/HEAD operations are direction http requests to file ids, e.g., `http://127.0
 | width  | if the stored file has ".png", ".jpg", ".jpeg", ".gif", apply resizing | empty |
 | height | if the stored file has ".png", ".jpg", ".jpeg", ".gif", apply resizing | empty |
 | mode   | if resizing, "fit", or "fill". Or just resizing, unless width==height, which default to thumbnail mode | empty |
+| crop_x1 | if the stored file has ".png", ".jpg", ".jpeg", ".gif", apply cropping, requires: 0 <= value < image width   | empty |
+| crop_y1 | if the stored file has ".png", ".jpg", ".jpeg", ".gif", apply cropping, requires: 0 <= value < image height | empty |
+| crop_x2 | if the stored file has ".png", ".jpg", ".jpeg", ".gif", apply cropping, requires: crop_x1 < value <= image width | empty |
+| crop_y2 | if the stored file has ".png", ".jpg", ".jpeg", ".gif", apply cropping, requires: crop_y1 < value <= image height | empty |
 
 | Request Header | Description | Default |
 | ---- | -- | -- |