Update README.md: add notification on COCO-Stuff download

Former-commit-id: 9c893b6bd9b61c40b16ee3bebb02eea8f7eb7cbe

README.md

@@ -111,6 +111,8 @@ The training data consists of 4 types of data:
 
 1. Semantic segmentation datasets: [ADE20K](http://data.csail.mit.edu/places/ADEchallenge/ADEChallengeData2016.zip), [COCO-Stuff](https://github.com/nightrome/cocostuff#downloads), [Mapillary](https://www.mapillary.com/dataset/vistas), [PACO-LVIS](https://github.com/facebookresearch/paco/tree/main#dataset-setup), [PASCAL-Part](https://github.com/facebookresearch/VLPart/tree/main/datasets#pascal-part)
 
+    Note: For COCO-Stuff, please use the annotation file [stuffthingmaps_trainval2017.zip](http://calvin.inf.ed.ac.uk/wp-content/uploads/data/cocostuffdataset/stuffthingmaps_trainval2017.zip).
+
 2. Referring segmentation datasets: [refCOCO](https://web.archive.org/web/20220413011718/https://bvisionweb1.cs.unc.edu/licheng/referit/data/refcoco.zip), [refCOCO+](https://web.archive.org/web/20220413011656/https://bvisionweb1.cs.unc.edu/licheng/referit/data/refcoco+.zip), [refCOCOg](https://web.archive.org/web/20220413012904/https://bvisionweb1.cs.unc.edu/licheng/referit/data/refcocog.zip), [refCLEF](https://web.archive.org/web/20220413011817/https://bvisionweb1.cs.unc.edu/licheng/referit/data/refclef.zip) ([saiapr_tc-12](https://web.archive.org/web/20220515000000/http://bvisionweb1.cs.unc.edu/licheng/referit/data/images/saiapr_tc-12.zip)) 
 
     Note: the origianl links of refCOCO series data are down, and we update them with new ones