Add dataset description to README

Author: HannaMao

README.md

@@ -95,6 +95,47 @@ Click the links below to download the checkpoint for the corresponding model typ
 * `vit_l`: [ViT-L SAM model.](https://dl.fbaipublicfiles.com/segment_anything/sam_vit_l_0b3195.pth)
 * `vit_b`: [ViT-B SAM model.](https://dl.fbaipublicfiles.com/segment_anything/sam_vit_b_01ec64.pth)
 
+## Dataset
+See [here](https://ai.facebook.com/datasets/segment-anything/) for an overview of the datastet. The dataset can be downloaded [here](https://ai.facebook.com/datasets/segment-anything-downloads/). By downloading the datasets you agree that you have read and accepted the terms of the SA-1B Dataset Research License.
+
+We save masks per image as a json file. It can be loaded as a dictionary in python in the below format.
+
+
+```python
+{
+    "image"                 : image_info,
+    "annotations"           : [annotation],
+}
+
+image_info {
+    "image_id"              : int,              # Image id
+    "width"                 : int,              # Image width
+    "height"                : int,              # Image height
+    "file_name"             : str,              # Image filename
+}
+
+annotation {
+    "id"                    : int,              # Annotation id
+    "segmentation"          : dict,             # Mask saved in COCO RLE format.
+    "bbox"                  : [x, y, w, h],     # The box around the mask, in XYWH format
+    "area"                  : int,              # The area in pixels of the mask
+    "predicted_iou"         : float,            # The model's own prediction of the mask's quality
+    "stability_score"       : float,            # A measure of the mask's quality
+    "crop_box"              : [x, y, w, h],     # The crop of the image used to generate the mask, in XYWH format
+    "point_coords"          : [[x, y]],         # The point coordinates input to the model to generate the mask
+}
+```
+
+Image ids can be found in sa_images_ids.txt which can be downloaded using the above [link](https://ai.facebook.com/datasets/segment-anything-downloads/) as well.
+
+To decode a mask in COCO RLE format into binary:
+```
+from pycocotools import mask as mask_utils
+mask = mask_utils.decode(annotation["segmentation"])
+```
+See [here](https://github.com/cocodataset/cocoapi/blob/master/PythonAPI/pycocotools/mask.py) for more instructions to manipulate masks stored in RLE format.
+
+
 ## License
 The model is licensed under the [Apache 2.0 license](LICENSE).