Add high quality synthetic face occlusion dataset link to readme. (open-mmlab#2453)

## Motivation

Add link for high quality synthetic face occlusion dataset.

## Modification

readme.md

Author: jinwonkim93

README.md

@@ -187,6 +187,7 @@ Supported datasets:
 - [x] [Potsdam](https://github.com/open-mmlab/mmsegmentation/blob/master/docs/en/dataset_prepare.md#isprs-potsdam)
 - [x] [Vaihingen](https://github.com/open-mmlab/mmsegmentation/blob/master/docs/en/dataset_prepare.md#isprs-vaihingen)
 - [x] [iSAID](https://github.com/open-mmlab/mmsegmentation/blob/master/docs/en/dataset_prepare.md#isaid)
+- [x] [High quality synthetic face occlusion](https://github.com/open-mmlab/mmsegmentation/blob/master/docs/en/dataset_prepare.md#delving-into-high-quality-synthetic-face-occlusion-segmentation-datasets)
 
 ## FAQ