Add status notice in README of prototype datasets (#7755)

Co-authored-by: Philip Meier <[email protected]>

Author: NicolasHug

torchvision/prototype/datasets/README.md

@@ -0,0 +1,7 @@
+# Status of prototype datasets
+
+These prototype datasets are based on [torchdata](https://github.com/pytorch/data)'s datapipes. Torchdata
+development [is
+paused](https://github.com/pytorch/data/#torchdata-see-note-below-on-current-status)
+as of July 2023, so we are not actively maintaining this module. There is no
+estimated date for a stable release of these datasets.