[Fix] Add notation of training setting of SegFormer (open-mmlab#1707)

* [Fix] Add notation of training setting of SegFormer

* fix

* fix

Author: MengzhangLI

configs/segformer/README.md

@@ -123,3 +123,5 @@ if args.aug_test:
 ```
 
 - Training of SegFormer is not very stable, which is sensitive to random seeds.
+
+- We use default training setting in MMSegmentation rather than `RepeatDataset` adopted in SegFormer official repo to accelerate [training](https://github.com/NVlabs/SegFormer/blob/master/local_configs/_base_/datasets/ade20k_repeat.py#L38-L39), here is its related [issue](https://github.com/NVlabs/SegFormer/issues/25).